Resources / Glossary
The email deliverability glossary.
Every term you'll meet when email gets serious, defined in plain language by people who operate this infrastructure daily.
- Email deliverability
- The practice of getting email accepted and placed in the inbox. A function of sender reputation, authentication, infrastructure, list quality, and content. Distinct from "delivery," which only means the receiving server accepted the message.
- Inbox placement
- Where an accepted message actually lands: inbox, promotions, or spam. Mailbox providers decide placement using reputation and engagement signals, so "delivered" does not mean "seen."
- Sender reputation
- The trust score mailbox providers assign to a sender, built from complaint rates, bounce rates, spam-trap hits, authentication, and history. It attaches to both IPs and domains.
- Domain reputation
- Reputation tied to the sending domain (and the domains in your links). It follows you across IPs, which is why changing IPs cannot fix a domain or content problem.
- IP reputation
- Reputation tied to a sending IP address. New IPs have none, which is why warming exists; damaged IPs face throttling or blocks until rehabilitated.
- IP warming (warmup)
- Gradually increasing volume on a new sending IP so mailbox providers can build trust. Ramping too fast triggers throttling and spam-foldering; done well, it earns durable capacity.
- Dedicated IP
- A sending IP used by a single sender, so its reputation reflects only that sender’s behavior. Best for steady, meaningful volume.
- Shared IP pool
- Sending IPs used by multiple senders, where reputation is collective. Quality of the pool, and the discipline of its operator, determines everyone’s outcomes.
- Email authentication
- The trio of standards (SPF, DKIM, DMARC) that prove a message genuinely comes from its claimed sender. Now a hard requirement for bulk senders at Gmail, Yahoo, and Microsoft.
- SPF (Sender Policy Framework)
- A DNS record listing which servers may send mail for your domain. Receivers check the connecting server against it.
- DKIM (DomainKeys Identified Mail)
- A cryptographic signature added to messages, verifiable via a public key in your DNS, proving the message wasn’t altered and is authorized by the signing domain.
- DMARC
- A policy layer on top of SPF and DKIM that tells receivers what to do when authentication fails, and provides reporting. Requires alignment between the visible From domain and the authenticated domain.
- DMARC alignment
- The requirement that the domain users see in From matches the domain that passed SPF or DKIM. Alignment is what makes authentication meaningful, and unaligned mail increasingly goes to spam.
- BIMI
- Brand Indicators for Message Identification. Displays your logo next to authenticated mail in supporting inboxes. Requires enforced DMARC.
- MTA-STS
- A standard letting domains require TLS for inbound mail delivery, protecting messages from downgrade attacks in transit.
- TLS-RPT
- Companion reporting standard to MTA-STS: receivers report TLS delivery failures so operators can detect interception or misconfiguration.
- Reverse DNS (PTR record)
- The DNS record mapping a sending IP back to a hostname. Receivers expect it to exist and make sense; generic or missing PTR records are a spam signal.
- Feedback loop (FBL)
- A mechanism where mailbox providers report spam complaints back to senders so the complaining recipients can be suppressed immediately.
- Complaint rate
- The share of delivered mail recipients mark as spam. Gmail’s published threshold is 0.3%, and sustained rates near it damage placement for everything you send.
- Bounce rate
- The share of sends rejected by receivers. High bounce rates signal poor list quality and damage sender reputation.
- Hard bounce
- A permanent delivery failure: the address or domain doesn’t exist. Hard-bounced addresses must be suppressed immediately.
- Soft bounce
- A temporary failure: full mailbox, busy server, rate limiting. Retried sensibly, these often deliver; retried carelessly, they look like abuse.
- Suppression list
- The list of addresses you must not mail: unsubscribes, hard bounces, complainers. Maintaining it isn’t optional. It’s both law and deliverability hygiene.
- DNC (Do-Not-Contact)
- A suppression list of specific recipients who must never be mailed again, typically from unsubscribes, complaints, or direct requests. Properly implemented, DNC is enforced at the sending infrastructure itself, so no upstream application can override it.
- DNP (Do-Not-Prospect)
- A suppression scope covering entire recipient domains or organizations that must not be prospected, often for legal, contractual, or relationship reasons. Enforced at the gate, it blocks outreach to anyone at the covered domain.
- Webhook
- An HTTP callback that pushes events (deliveries, bounces, deferrals, suppression hits) to your systems in near real time. The standard way sending infrastructure feeds CRMs, dashboards, and automated workflows.
- List hygiene
- The ongoing removal of dead, invalid, and unengaged addresses from your lists. The single highest-leverage deliverability habit.
- Spam trap
- An address that exists to catch senders with poor list practices. Hitting traps signals purchased or stale lists and can trigger blocklisting. Avoided by consent-based collection and hygiene.
- Blocklist (blacklist)
- A published list of IPs or domains accused of sending abuse, consulted by receivers to refuse mail. Examples include Spamhaus lists.
- Delisting
- The process of getting removed from a blocklist, typically requiring evidence, remediation, and the complete rejection details from affected sends.
- Throttling / rate limiting
- Receivers slowing a sender down (temporary deferrals) when volume or reputation exceeds comfort. Respecting per-provider tolerances avoids most of it.
- Greylisting
- A receiver tactic of temporarily rejecting first contact from unknown senders; legitimate mail servers retry and pass, while crude spamware often doesn’t.
- Transactional email
- Mail triggered by a user action or account relationship: resets, receipts, OTPs, notifications. The highest-stakes category: expected, wanted, and time-sensitive.
- Operational email
- Service and system mail that keeps a business running: alerts, status updates, invoices, reminders. Like transactional mail, it must arrive reliably.
- SMTP relay
- A service that accepts your mail and delivers it onward to recipients. Quality varies enormously, and the relay’s reputation practices become yours.
- Email API
- Programmatic sending and management interfaces: injecting mail, provisioning domains and mailboxes, and consuming delivery events from your own software.
- List-Unsubscribe / one-click unsubscribe
- Headers that let recipients unsubscribe directly from their mail client. Mandatory for bulk senders under Gmail and Yahoo requirements; honoring them quickly protects complaint rates.
- Postmaster tools
- Provider-supplied dashboards (such as Google Postmaster Tools) exposing your reputation, authentication, and spam-rate data as that provider sees it.
- Seed testing
- Sending to a controlled panel of test addresses across providers to measure inbox placement before and during campaigns.
- Engagement-based filtering
- Modern placement logic that weighs how recipients interact with your mail (opens, replies, deletes-without-reading) alongside technical signals.
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