Email Deliverability Glossary
39 plain-language definitions of terms you'll encounter when managing email deliverability — from authentication standards to ISP filtering metrics.
BIMI (Brand Indicators for Message Identification)
A standard that displays your verified brand logo next to emails in supported inbox clients. Requires DMARC at p=quarantine or p=reject. Certificate requirements vary by provider: Gmail requires a VMC (Verified Mark Certificate) or CMC (Common Mark Certificate) from an approved Certificate Authority; Yahoo Mail supports BIMI without a VMC. VMCs require a registered trademark.
Blocklist (Blacklist / DNSBL)
A database of IP addresses or domains known for sending spam. ISPs check blocklists when deciding whether to accept email. Major blocklists include Spamhaus (covering IPs via SBL/ZEN and domains via DBL), Barracuda, and Invaluement. Listing causes significant delivery problems and typically requires a formal delisting request to the blocklist operator.
Bot Activity / Click Bot
Automated systems that open emails and click links without human involvement. Common in corporate email security scanning and spam filter testing. Bot activity inflates open and click rates, distorting engagement metrics used by ISPs to assess sender reputation. InboxEagle detects bot activity with 98.4% accuracy.
CAN-SPAM Act
The US law governing commercial email. Key requirements: accurate header information, truthful subject lines, physical address in every email, clear opt-out mechanism, and honoring unsubscribe requests within 10 business days.
CAN-SPAM Act ExplainedCASL (Canada's Anti-Spam Legislation)
Canada's email law, stricter than CAN-SPAM. Requires express or implied consent before sending commercial email, proper identification, and unsubscribe mechanisms. Violations can result in fines up to $10 million CAD.
CASL Compliance GuideDedicated IP
A sending IP address used exclusively by one sender. Reputation reflects only that sender's behavior. Recommended for senders above 100,000 emails per month for full reputation control. Below that volume, shared IPs are often better.
Shared vs Dedicated IP — Full GuideDelivery Rate
The percentage of emails accepted by the receiving mail server. Different from inbox placement rate — a 99% delivery rate doesn't mean 99% inbox placement. Emails can be accepted and then routed to spam.
DKIM (DomainKeys Identified Mail)
An email authentication standard that adds a cryptographic signature to outgoing emails. Receiving servers verify the signature using a public key in your DNS, proving the email wasn't modified in transit and that it originated from your domain. Use 2048-bit keys — 1024-bit keys are considered weak and are being phased out by major providers.
DMARC (Domain-based Message Authentication, Reporting & Conformance)
An email policy protocol that ties SPF and DKIM together. It tells receiving servers what to do when authentication fails (p=none, p=quarantine, p=reject) and sends aggregate reports to the sender. For DMARC to pass, at least one of SPF or DKIM must both authenticate and pass alignment.
DMARC Aggregate Report (RUA)
A daily XML report sent by ISPs to the rua= email address in your DMARC record. Shows which IPs sent mail claiming to be from your domain, with SPF/DKIM pass/fail rates. InboxEagle parses and visualizes these automatically.
DMARC Alignment
The requirement that the domain in the email's From: header matches the domain authenticated by SPF or DKIM. In relaxed alignment (the default), organizational domain matches are accepted — mail.example.com aligns with example.com. In strict alignment, the domains must match exactly. DMARC fails if neither SPF nor DKIM achieves alignment, even if both authentication checks individually pass. Alignment failures are among the most common reasons legitimate email fails DMARC.
DMARC p=none
The DMARC monitoring policy. Emails that fail authentication are still delivered, but aggregate reports are sent to the rua= address. The starting policy for all new DMARC implementations.
DMARC p=quarantine
The DMARC enforcement policy that routes failing messages to the spam folder. Intermediate step between p=none and p=reject during DMARC rollout.
DMARC p=reject
The strictest DMARC policy. Messages that fail authentication are blocked entirely — they never reach the recipient. Maximum protection against spoofing and phishing.
Domain Warming
Similar to IP warming but for a new sending domain. ISPs have no history for new domains and treat them with extra suspicion. Volume should be ramped gradually with highly engaged subscribers first.
IP & Domain Warming GuideDouble Opt-In
A signup process where new subscribers receive a confirmation email and must click to verify their address before being added to the list. Produces higher list quality and lower complaint rates compared to single opt-in.
Email Service Provider (ESP)
A platform used to send bulk marketing emails. Examples include Klaviyo, Mailchimp, Sendgrid, AWS SES, and HubSpot. ESPs provide sending infrastructure, list management, and campaign analytics.
Engagement-Based Segmentation
The practice of grouping subscribers by their recent engagement activity (opens, clicks, purchases) and sending only to active segments. The single most effective tactic for maintaining sender reputation and inbox placement.
Engagement-Based Segmentation GuideFeedback Loop (FBL)
A service where ISPs send spam complaint notifications back to the sending ESP. When a recipient marks an email as spam, an ARF (Abuse Reporting Format) report is sent so the sender can suppress that address. Yahoo, Microsoft, and Comcast offer traditional FBLs. Gmail does not send individual complaint reports — Google Postmaster Tools is Gmail's equivalent, providing aggregated spam rate data rather than per-email notifications.
GDPR (General Data Protection Regulation)
The EU's data privacy regulation affecting email marketing. Requires explicit consent for email marketing, the right to erasure, and data processing transparency. Applies to any sender with EU subscribers regardless of where the sender is located.
GDPR & Email Marketing GuideGoogle Postmaster Tools
A free tool from Google that shows senders how Gmail views their domain — including domain reputation (rated Low, Medium, High, or Very High), IP reputation, spam complaint rate, and authentication compliance. Requires domain ownership verification via DNS. Data has a 24–48 hour delay and is aggregated. Also surfaces a Spam Rate dashboard that tracks your complaint rate against Gmail's 0.10% and 0.30% enforcement thresholds.
Hard Bounce
A permanent email delivery failure — the address doesn't exist, the domain is invalid, or the recipient's server has permanently rejected the message. Hard bounces should be removed from lists immediately.
Inbox Placement Rate (IPR)
The percentage of emails that land in the recipient's inbox folder, as opposed to the spam folder, promotions tab, or being silently blocked. A healthy inbox placement rate is above 95%. Measured using seed list testing.
IP Warming
The process of gradually increasing sending volume from a new IP address over 4–8 weeks, allowing ISPs to build a reputation history before full volume is sent. Skipping IP warming causes spam filtering due to lack of reputation.
IP & Domain Warming GuideISP (Internet Service Provider) / Mailbox Provider
Companies that provide email inboxes to consumers. The major mailbox providers are Gmail (Google), Outlook/Hotmail/Live (Microsoft), Yahoo Mail, and Apple Mail. Each runs its own spam filtering system.
List Hygiene
The practice of keeping an email list clean by removing invalid addresses, hard bounces, spam complainers, and inactive subscribers. Good list hygiene is fundamental to maintaining sender reputation.
Mail Privacy Protection (MPP)
Apple's feature (iOS 15+, macOS Monterey+) that pre-fetches email content and loads tracking pixels before the user opens the message. This records opens as happening regardless of whether the user actually opens the email, inflating open rates for Apple Mail users.
One-Click Unsubscribe
A machine-readable unsubscribe mechanism required by Gmail and Yahoo for bulk senders since February 2024. Implemented via the List-Unsubscribe and List-Unsubscribe-Post headers (RFC 8058). When a recipient clicks 'Unsubscribe' in their inbox interface, a POST request is sent directly to the sender's endpoint — no email or page visit required. Required for senders of 5,000 or more commercial messages per day; requests must be processed within 2 days. Non-compliance increases spam complaint rates as users resort to the spam button instead.
One-Click Unsubscribe GuideRe-engagement Campaign
A campaign sent to inactive subscribers asking if they still want to receive email. Subscribers who don't engage with the re-engagement campaign are suppressed. Used before sunset policies take effect.
Seed List
A set of real test email addresses at major ISPs and email clients used to test inbox placement. Sending to seed list addresses shows exactly where email lands (inbox, spam, promotions) at each provider before sending to real subscribers.
Sender Reputation
A trust score maintained by ISPs (Gmail, Outlook, Yahoo) for your sending domain and IP addresses. Based on spam complaint rates, bounce rates, engagement signals, and sending history. Difficult to build, easy to damage.
Soft Bounce
A temporary email delivery failure — the mailbox is full, the server is temporarily unavailable, or the message was too large. Multiple consecutive soft bounces from the same address should trigger suppression.
Spam Complaint Rate
The percentage of recipients who mark your email as spam. Gmail's thresholds: below 0.10% is acceptable, 0.10%–0.30% triggers a warning, above 0.30% triggers active filtering. The highest-weighted signal in sender reputation. Yahoo enforces similar thresholds.
Spam Trap
An email address used by ISPs and blocklist operators to identify senders who don't follow best practices. Pristine traps were never valid addresses. Recycled traps are abandoned addresses reactivated as traps. Hitting spam traps causes blocklisting.
Avoiding Spam Traps — Full GuideSPF (Sender Policy Framework)
A DNS TXT record that lists all IP addresses and services authorized to send email for your domain. ISPs check SPF to verify the sending IP is legitimate. SPF is limited to 10 DNS-querying mechanisms (include, a, mx, ptr, exists) — exceeding this causes a PermError that most receivers treat as an authentication fail. The all qualifier controls failure handling: ~all (softfail, still delivers to inbox or spam) or -all (hardfail, rejects the message). Note: the ptr mechanism is deprecated per RFC 7208 and should never be used.
Sunset Policy
An automated rule that removes or suppresses subscribers who haven't engaged with email after a set time period (typically 6–12 months). Sunsetting inactive subscribers protects sender reputation by improving engagement rates.
Transactional Email
Emails triggered by a specific user action — order confirmations, password resets, shipping notifications. Subject to different deliverability standards than marketing email. Should be sent from a separate subdomain and IP to protect the transactional stream's high deliverability.
Yahoo Sender Hub
Yahoo Mail's sender monitoring tool, equivalent to Google Postmaster Tools. Provides spam complaint rate data, bounce reports, and domain reputation signals for senders mailing to Yahoo Mail addresses. Requires domain ownership verification and DKIM-authenticated mail to access complaint data.
Related Guides
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