Reference
Email deliverability glossary — 69 terms.
From DMARC and DKIM to sender reputation, bounce handling, and inbox placement — the authoritative reference for email marketers, developers, and deliverability engineers.
A
- Authentication
Authentication (Email)
The collective set of protocols — SPF, DKIM, and DMARC — that verify a sender is who they claim to be. Mailbox providers use authentication to decide whether an email is legitimate or spoofed. Without proper authentication, emails are far more likely to be filtered or rejected. See the complete authentication guide →
- Testing
A/B Testing (Email)
The practice of sending two variants of an email to different audience segments to determine which performs better. Common variables include subject lines, from names, send times, and content layout. A/B testing optimises content performance but cannot diagnose or fix deliverability failures — those require signal monitoring.
- Filtering
Allowlist (Whitelist)
A list of pre-approved sender addresses or IP addresses that a mailbox provider or recipient organisation has designated as trusted. Emails from allowlisted senders bypass most spam filters. Adding a sender to contacts is the most common form of allowlisting and is a strong positive engagement signal used by Gmail to improve inbox placement.
B
- Reputation
Blacklist (Email Blacklist)
A database of IP addresses or domains known to send spam, maintained by organisations like Spamhaus, SORBS, and Barracuda. If your sending IP or domain is listed, mailbox providers reject or heavily filter your emails. Monitoring blacklist status is a core function of InboxStack Brain. Blacklist removal guide →
- Bounces
Bounce — Hard
A permanent delivery failure where the recipient address does not exist, is disabled, or has explicitly rejected the sender. Hard bounces must be suppressed immediately. A hard bounce rate above 2% triggers automatic throttling or blocking by most mailbox providers and signals poor list hygiene.
- Bounces
Bounce — Soft
A temporary delivery failure caused by a full mailbox, a temporarily unavailable server, or message size limits. Soft bounces typically resolve on retry. Repeated soft bounces from the same address over time should be treated as hard bounces and suppressed to protect sender reputation.
- Metrics
Bounce Rate
The percentage of sent emails that were not delivered successfully. Bounce rate = (bounced emails / sent emails) × 100. Keep hard bounce rate below 2%. Rates above this trigger automatic filtering across all mailbox providers. High bounce rates are a leading indicator of deliverability failure — InboxStack Brain surfaces bounce anomalies in real time.
- Authentication
BIMI (Brand Indicators for Message Identification)
An email standard that allows brands to display a verified logo next to their emails in supporting mailbox clients (Gmail, Yahoo, Apple Mail). BIMI requires a valid DMARC policy at quarantine or reject, and typically a Verified Mark Certificate (VMC). It signals trustworthiness to both MBPs and recipients. BIMI setup guide →
C
- Compliance
CAN-SPAM Act
A US law governing commercial email. Requirements include a physical mailing address, a clear unsubscribe mechanism honoured within 10 business days, honest subject lines, and no deceptive headers. CAN-SPAM applies to commercial messages — not transactional emails. Violations carry fines up to $51,744 per email.
- Metrics
Complaint Rate
The percentage of recipients who mark your email as spam. Google recommends keeping complaint rates below 0.10% — rates above 0.30% cause severe inbox placement problems. InboxStack Brain pulls complaint rate data directly from Google Postmaster Tools and Microsoft SNDS in real time, alerting when thresholds are approached.
- Sending
Cold Email
Outreach sent to recipients who have not previously opted in to the sender. Cold email has unique deliverability challenges — no prior engagement history, no permission signal. Authentication, limited volume, high personalisation, and careful list sourcing are critical. Cold email deliverability playbook →
- Filtering
Content Filter
An algorithm that analyses email content — subject lines, body text, HTML, images, and links — to determine spam likelihood. Triggers include spammy phrases, misleading links, and poor text-to-image ratios. Content filtering is one layer among many; reputation signals carry far more weight in modern MBP filtering systems.
- DNS
CNAME Record
A DNS record type that maps an alias to a canonical domain name. In email, CNAME records are used for DKIM key publishing (some ESP implementations), click-tracking domains, and open-tracking pixel domains. Using a subdomain with CNAME for tracking helps isolate reputation from the root sending domain.
D
- Authentication
DMARC (Domain-based Message Authentication, Reporting & Conformance)
An email authentication protocol that builds on SPF and DKIM to tell mailbox providers what to do when authentication fails: none (monitor only), quarantine (send to spam), or reject (block entirely). DMARC also enables reporting — giving senders visibility into authentication failures and domain abuse. A DMARC policy at reject is required for BIMI and is mandatory for bulk senders to Gmail and Yahoo.
- Authentication
DKIM (DomainKeys Identified Mail)
An authentication method that adds a cryptographic signature to email headers, tied to the sending domain. The recipient's server verifies the signature using a public key in DNS. DKIM proves the message was not altered in transit and that the signing domain endorses it. It is one of the two mechanisms checked for DMARC alignment.
- Infrastructure
DNS (Domain Name System)
The internet's directory system mapping domain names to IP addresses and storing email authentication records. Critical DNS record types for deliverability: MX (mail routing), SPF (TXT), DKIM (TXT), DMARC (TXT under _dmarc), and optionally BIMI and MTA-STS. DNS misconfiguration is among the most common causes of deliverability failures.
- Reputation
Domain Reputation
The trustworthiness score assigned to a sending domain by mailbox providers, separate from IP reputation. Based on sending history, complaint rates, engagement, authentication compliance, and blacklist status. Domain reputation persists across IP changes — making it one of the most critical long-term deliverability assets to protect.
- Metrics
Deliverability Rate
The percentage of sent emails accepted by the receiving mail server (not bounced). Important: deliverability rate ≠ inbox placement rate. An email can be delivered to a server but still land in spam. Inbox placement rate is the more meaningful metric for understanding whether recipients actually see your mail.
- List Management
Double Opt-in (Confirmed Opt-in)
A subscription process where a new subscriber confirms their address by clicking a link in a confirmation email. Double opt-in eliminates invalid addresses, reduces spam traps, and produces highly engaged lists. It is the strongest signal of subscriber intent and directly improves long-term sender reputation.
E
- Warming
Email Warm-up
The process of gradually increasing sending volume from a new domain or IP to build positive reputation with mailbox providers. A typical schedule starts with tens of emails per day and scales over 4–8 weeks, prioritising engaged recipients first. Skipping warm-up causes immediate spam filtering. Full warm-up guide →
- Authentication
Email Authentication
The combination of SPF, DKIM, and DMARC that verify email sender legitimacy. Authentication is table stakes for inbox delivery — without it, modern mailbox providers will reject or heavily filter messages, particularly from bulk senders. All three protocols must be correctly configured and aligned for DMARC compliance.
- Metrics
Engagement Rate
A composite measure of how recipients interact with emails: opens, clicks, replies, and forwards — weighted against spam complaints and unsubscribes. Gmail uses per-recipient engagement signals to determine inbox placement. Low engagement accelerates deliverability decline. Sending only to engaged subscribers is the fastest recovery lever.
- Infrastructure
ESP (Email Service Provider)
A platform that handles sending, delivery, and tracking of email campaigns — examples include Mailchimp, SendGrid, Postmark, and Amazon SES. ESPs manage IP pools, bounce processing, and analytics. Deliverability responsibility is shared: ESPs handle infrastructure reputation, senders are responsible for domain reputation and list quality.
F
- Reputation
Feedback Loop (FBL)
A service offered by mailbox providers (Yahoo, Outlook) that notifies senders when a recipient marks their email as spam. FBL reports enable immediate suppression of complainers, limiting reputation damage. Google Postmaster Tools provides complaint rate data as Gmail's equivalent. InboxStack Brain ingests complaint data from all major MBPs automatically.
- Identity
From Address
The email address displayed as the sender. The From domain must align with the DKIM signing domain for DMARC compliance. An inconsistent or mismatched From address undermines authentication coverage and erodes recipient trust. Using a stable From address tied to your authenticated domain is a deliverability fundamental.
- Filtering
Spam Filter
A system used by mailbox providers to classify inbound email as legitimate or spam. Modern spam filters are machine learning models evaluating hundreds of signals — reputation, authentication, content, engagement history, and sending patterns — rather than simple keyword lists. No single trick defeats them; consistent good practices are what build lasting inbox access.
G
- Compliance
GDPR (General Data Protection Regulation)
EU regulation governing how personal data — including email addresses — is collected, stored, and used. For email, GDPR requires a lawful basis for processing (typically explicit consent for marketing), easy unsubscribe mechanisms, and data retention limits. Fines run up to €20 million or 4% of global annual revenue.
- Monitoring
Google Postmaster Tools
A free Google tool providing Gmail-specific deliverability data: domain reputation, IP reputation, spam rate, authentication pass rates, and delivery errors. Requires domain verification and only shows data for domains with significant Gmail volume. InboxStack Brain integrates Postmaster Tools data into a real-time unified dashboard alongside Outlook and Yahoo signals. Google & Yahoo sender requirements →
- Filtering
Greylisting
A spam-filtering technique where a mail server temporarily rejects a first email from an unknown sender with a 4xx code. Legitimate servers retry; spam bots typically do not. Greylisting can add minutes to delivery and affects time-sensitive transactional messages. It is transparent to senders but shows up as temporary failures in MTA logs.
H
- Bounces
Hard Bounce
See Bounce — Hard. A permanent delivery failure requiring immediate list suppression. Hard bounces signal poor list hygiene to mailbox providers, triggering stricter filtering across all messages from that sender — not just to the bounced address.
- Infrastructure
Email Header
Metadata prepended to an email message containing routing info, authentication results, timestamps, and identifiers. Key fields: From, To, Subject, Message-ID, Received (hop-by-hop path), DKIM-Signature, and Authentication-Results. Analysing headers is essential for diagnosing delivery failures — InboxStack Brain's RCA Inspector surfaces header anomalies automatically.
- List Management
Honeypot (Spam Trap)
See Spam Trap. Email addresses created to identify senders with poor list hygiene. Honeypots are never used by real people, so any email sent to one is evidence of list-buying, scraping, or failure to sunset inactive addresses. Hitting honeypots causes immediate blacklisting.
I
- Metrics
Inbox Placement Rate
The percentage of delivered emails that land in the recipient's primary inbox — not spam, junk, or a promotional tab. Unlike deliverability rate (any server acceptance), inbox placement rate reflects actual visibility. Industry benchmark is 85%+ for healthy senders. InboxStack Brain tracks inbox placement rates across Gmail, Outlook, and Yahoo simultaneously. What is inbox placement? → · Benchmarks by provider → · Inbox placement vs deliverability →
- Infrastructure
IP Address (Sending IP)
The internet protocol address used by a mail server to send email. Mailbox providers assign reputation scores to sending IPs based on history, complaint rates, and authentication compliance. Shared IPs spread reputation across multiple senders; dedicated IPs give full control — and full responsibility.
- Reputation
IP Reputation
The trustworthiness score assigned to a specific sending IP by mailbox providers and blacklist operators. Influenced by complaint rates, bounce rates, spam trap hits, volume consistency, and blacklist status. IP reputation can recover but typically takes 4–8 weeks of clean sending. InboxStack Brain monitors IP reputation continuously via Google Postmaster and SNDS.
- Warming
IP Warming
The process of building reputation for a new sending IP by gradually ramping volume over 4–8 weeks. New IPs have no history so MBPs treat them cautiously. Proper IP warming prioritises engaged recipients first and avoids volume spikes. See also Email Warm-up.
J
- Filtering
Junk Folder
The Microsoft Outlook and Windows Mail equivalent of the spam folder. Emails land here when Microsoft's filters classify them as unwanted. Microsoft's filtering is informed by SNDS (Smart Network Data Services), complaint data, and proprietary reputation signals — all monitored by InboxStack Brain's Radar Monitor in real time.
L
- List Management
List Hygiene
Regularly cleaning an email list by removing invalid addresses, hard bounces, spam traps, and long-term inactive subscribers. Good hygiene keeps bounce rates low and engagement high. Lists should be validated at collection and re-evaluated monthly for active senders. Full list hygiene guide →
- List Management
List Segmentation
Dividing an email list into sub-groups based on attributes like engagement history, geography, product usage, or subscription source. Segmentation improves engagement rates and reduces complaints by matching content to audience interest. Sending only to highly engaged segments is one of the fastest ways to recover from deliverability problems.
M
- Infrastructure
MBP (Mailbox Provider)
A company providing email mailbox services to end users. The three dominant MBPs are Google (Gmail, ~40% market share), Microsoft (Outlook/Hotmail, ~30%), and Yahoo (~15%). Each has its own filtering infrastructure, reputation systems, and deliverability tools. InboxStack Brain monitors deliverability across all three simultaneously, surfacing MBP-specific divergence instantly.
- DNS
MX Record (Mail Exchange Record)
A DNS record specifying which mail server receives email for a domain. MX records include a priority value — lower numbers are tried first. Misconfigured MX records prevent email delivery entirely. Checking MX records is a fundamental step in any deliverability audit. Deliverability audit checklist →
- Infrastructure
MTA (Mail Transfer Agent)
Software transferring email between servers via SMTP — examples include Postfix, Exim, and Sendmail. The MTA handles queuing, retry logic, bounce classification, and DKIM signing. MTA configuration directly affects delivery speed, bounce accuracy, and authentication success rates.
- Monitoring
Machine Learning (Email Deliverability)
The application of ML models to deliverability data — detecting anomalies in inbox rates, predicting reputation drops before they happen, and classifying the root cause of placement failures. ML-driven monitoring catches the gradual signal decay that threshold-based alerting misses. InboxStack Brain's Signal Engine is built on this approach. How AI & ML are changing deliverability →
N
- Sending
Newsletter
A regularly scheduled email to a subscribed audience, typically containing editorial content, updates, or curated resources. Newsletters depend heavily on consistent engagement to maintain inbox placement. Declining open rates signal disengagement to MBPs. Re-engagement campaigns and sunset policies for inactive subscribers are essential newsletter hygiene practices.
O
- Metrics
Open Rate
The percentage of delivered emails opened by recipients. Note: Apple Mail Privacy Protection (MPP) inflates open rates by pre-loading tracking pixels — making raw open rates increasingly unreliable. Click rate and reply rate are more accurate engagement indicators for deliverability monitoring purposes.
- List Management
Opt-in
The process by which a recipient agrees to receive emails. Single opt-in begins sending immediately after address collection. Double opt-in requires a confirmation click. Opt-in quality directly determines list hygiene and engagement — purchased or scraped lists that bypass opt-in are a primary cause of spam complaints and blacklisting.
- List Management
Opt-out (Unsubscribe)
The mechanism by which recipients remove themselves from a mailing list. CAN-SPAM requires processing within 10 business days; GDPR requires it to be as easy as subscribing. Gmail's one-click unsubscribe (RFC 8058) is now mandatory for bulk senders. Honour unsubscribes immediately — delayed processing causes spam complaints.
P
- DNS
PTR Record (Reverse DNS)
A DNS record mapping an IP address back to a domain name. Mailbox providers use PTR records to verify that a sending IP is associated with a legitimate domain. Missing or mismatched PTR records are a common cause of delivery failures, particularly to Microsoft's email infrastructure, which performs strict reverse DNS checks.
- Security
Phishing
A cyberattack using fraudulent emails that impersonate legitimate senders to steal credentials or install malware. DMARC at reject is the primary defence against domain spoofing used in phishing. Domains without DMARC enforcement are actively exploited — InboxStack Brain alerts immediately when DMARC policy is missing or insufficiently strict.
- Sending
Plain Text Email
An email with no HTML formatting — just text. Most ESPs send multipart MIME with both HTML and plain text versions. A quality plain text alternative improves deliverability (some filters penalise HTML-only emails), accessibility, and rendering in environments where HTML is disabled or stripped.
R
- Reputation
Sender Reputation
The overall trustworthiness score assigned to a sender by mailbox providers, based on IP reputation, domain reputation, complaint rates, bounce rates, engagement history, and authentication compliance. Sender reputation is the single largest factor in inbox placement decisions. Full sender reputation guide →
- Infrastructure
Return-Path (Envelope Sender)
The email address to which bounce notifications are sent — also called MAIL FROM. The Return-Path domain is what SPF authenticates. It must match or align with the From domain for DMARC SPF alignment. Many ESPs use their own Return-Path domains, which can create alignment issues if not configured correctly.
- Intelligence
RCA (Root Cause Analysis)
The process of identifying the underlying cause of a deliverability failure rather than its symptoms. InboxStack Brain's RCA Inspector uses a causal graph engine to trace signal chains automatically — for example, identifying that a Gmail spam rate spike was caused by a specific campaign segment with low engagement, not a content or authentication issue.
S
- Authentication
SPF (Sender Policy Framework)
A DNS-based protocol specifying which IP addresses and mail servers are authorised to send email on behalf of a domain. Receiving servers check the SPF record of the Return-Path domain. SPF alone does not prevent From address spoofing — DMARC is required for full protection. SPF records have a 10-DNS-lookup limit that is easy to exceed with multiple ESPs.
- Reputation
Sender Score
A 0–100 reputation metric assigned to sending IPs by Validity (formerly Return Path). A score above 80 is associated with good deliverability; below 70 indicates significant problems. Many ISPs and spam filters use Sender Score as a filtering input. InboxStack Brain tracks Sender Score as part of its continuous domain health monitoring.
- Filtering
Spam
Unsolicited bulk email sent without recipient consent or to disengaged recipients. The filtering definition is broader than legal definitions — an email can be CAN-SPAM compliant but classified as spam by Gmail based on low engagement and complaint signals. Relevance and consent are the ultimate spam avoidance strategies.
- List Management
Spam Trap
An email address used by blacklist operators and ISPs to identify poor-practice senders. Pristine traps were never used by real people — hitting one means list-buying or scraping. Recycled traps were real addresses that went abandoned — hitting one means failure to sunset inactive subscribers. Both types cause blacklisting and severe reputation damage.
- List Management
Suppression List
A database of email addresses that must never receive emails from a sender — hard bounces, spam complainers, and explicit unsubscribes. ESPs maintain suppression automatically; senders on their own infrastructure must implement it explicitly. Sending to suppressed addresses is both a deliverability failure and a compliance violation under CAN-SPAM and GDPR.
- Sending
Subject Line
The text displayed in the recipient's inbox before opening. Subject lines influence open rates and can trigger spam filters with deceptive claims or excessive capitalisation. However, subject line optimisation is a secondary concern — a strong sender reputation is what gets your email seen. A weak reputation means it never reaches the inbox regardless of subject.
T
- Security
TLS (Transport Layer Security)
An encryption protocol securing email in transit between mail servers. Most modern infrastructure supports opportunistic TLS (STARTTLS). MTA-STS enforces TLS and prevents downgrade attacks. Lack of TLS support is flagged by Google Postmaster Tools and negatively affects domain reputation scoring.
- Sending
Transactional Email
Automated one-to-one emails triggered by a user action — purchase receipts, password resets, shipping notifications, account alerts. Transactional emails have high engagement and should be sent from a separate subdomain or IP pool from marketing emails to protect their deliverability from campaign-related reputation fluctuations.
- Infrastructure
Throttling
A rate-limiting measure used by mailbox providers that limits email acceptance from a sending source, returning 4xx temporary failure codes. Throttling does not permanently reject messages but signals reputation problems. InboxStack Brain's Signal Engine detects throttling patterns early — before they escalate to blocking — and surfaces them as incidents.
U
- Metrics
Unsubscribe Rate
The percentage of recipients who opt out after receiving an email. A rate above 0.5% is a warning sign of list quality or content relevance issues. When unsubscribing is difficult, recipients complain instead — making an easy unsubscribe process essential for complaint rate management. Gmail's one-click unsubscribe is now mandatory for bulk senders.
- Risk
URL Shorteners (Deliverability Risk)
Services like bit.ly that obscure link destinations. Spammers use shortened URLs to hide malicious destinations, so many spam filters flag them as negative signals. Use full, transparent URLs from your own domain in email campaigns. For click tracking, use your own subdomain configured in your ESP settings.
V
- Sending
Volume (Sending Volume)
The number of emails sent per day, week, or campaign. Volume must match established reputation — sudden spikes trigger filtering even for good senders. Volume increases should be gradual, especially from new domains or IPs. Consistent, predictable volume builds reputation faster than erratic sending patterns.
- List Management
Email Verification
The process of checking whether an email address is valid and able to receive mail before sending. Verification services use SMTP handshakes, syntax checks, and domain validation to flag invalid, risky, or disposable addresses. Verifying lists at collection and before major campaigns reduces bounce rates and prevents spam trap hits.
W
- Infrastructure
Webhook
An HTTP callback sending real-time event data between systems — for example, an ESP pushing bounce and complaint events to a CRM as they happen. Webhook integration for bounce and complaint handling is essential for real-time list hygiene without manual exports. InboxStack Brain can push incident and signal events via webhook to external monitoring systems.
- Filtering
Whitelist
See Allowlist. A list of approved senders whose emails bypass spam filtering. Recipient-level whitelisting (adding a sender to contacts) is a strong positive engagement signal that mailbox providers use to improve inbox placement for that sender across similar recipients.
Z
- List Management
Zero-Bounce Policy (Hard Bounce Suppression)
The practice of permanently suppressing any address that returns a hard bounce — zero tolerance, no retries. Zero-bounce policy keeps bounce rates minimal and signals high list quality to mailbox providers. It is the baseline expectation for any sender with more than a few thousand recipients.
Monitor it all
Track every signal in real time.
Brain monitors reputation, authentication, engagement, blacklists, and MBP-specific signals across Gmail, Outlook & Yahoo — and tells you exactly what to fix when something moves.