Core Email Glossary
Canonical one-page definitions of the terms every sender should know: authentication, infrastructure, and deliverability, each linking to its full guide.
24 guides
SPF (Sender Policy Framework)
SPF is a DNS record that lists which mail servers are allowed to send email as your domain. A definition, with a link to the full guide.
DKIM (DomainKeys Identified Mail)
DKIM cryptographically signs each email so receivers can verify it is really from your domain and was not altered. A definition, with the full guide linked.
DMARC
DMARC tells receivers what to do when SPF or DKIM fails and reports who sends mail in your name. A definition, with the in-depth guide linked.
ARC (Authenticated Received Chain)
ARC preserves SPF and DKIM results when email is forwarded, so the final receiver can still trust authentication that a hop would otherwise break.
BIMI (Brand Indicators for Message Identification)
BIMI displays your verified brand logo next to authenticated email. It requires an enforced DMARC policy first.
SMTP (Simple Mail Transfer Protocol)
SMTP is the protocol mail servers use to transfer email. A definition of the handshake and reply codes, with the full guide linked.
DNS Records for Email
DNS records (MX, PTR, SPF, DKIM, DMARC) are where every deliverability check begins. A definition, with the full reference linked.
PTR Record (Reverse DNS)
A PTR record maps a sending IP back to a hostname. Receivers check it early in the SMTP handshake, and a missing PTR is a trust penalty.
MX Record
An MX record tells other servers where to deliver mail for your domain. It governs where replies to your cold email land.
Sender Reputation
Sender reputation is the trust score receivers assign your domain and IP. A definition, with the full guide linked.
Spam Score
A spam score is a filter's numeric estimate of how much your message looks like spam. A definition, with target numbers in the full guide.
Inbox Placement
Inbox placement is where an accepted message lands: inbox, Promotions, or spam. A definition, distinct from deliverability, with the guide linked.
Email Bounce (Hard and Soft)
A bounce is a message a receiving server refused. A definition of hard vs soft bounces, with the full guide linked.
Email Warm-up
Email warm-up is gradually ramping a new domain's sending volume to build reputation. A definition, with the full guide linked.
ESP (Email Service Provider)
An ESP is a service that sends email on your behalf. The term spans marketing platforms, transactional APIs, and cold outreach engines.
Click-Through Rate vs. Click-to-Open Rate
Click-Through Rate vs. Click-to-Open Rate — Teams compare denominators incorrectly across reports.
Delivery Latency: Measuring Time to Recipient Acceptance
Delivery Latency: Measuring Time to Recipient Acceptance — Teams need to distinguish instant delivery from eventual acceptance.
Delivered, Accepted, Deferred, and Bounced: Denominator Rules
Delivered, Accepted, Deferred, and Bounced: Denominator Rules — Operators use delivery, acceptance, and bounce as interchangeable outcomes.
Authentication-Results Header: Reading Receiver Assertions
Authentication-Results Header: Reading Receiver Assertions — Analysts need to read receiver-side SPF, DKIM, and DMARC results.
Greylisting: Temporary SMTP Deferral by the Receiver
Greylisting: Temporary SMTP Deferral by the Receiver — Senders see repeated 4xx responses and do not know whether retries work.
List-Unsubscribe Headers and One-Click Semantics
List-Unsubscribe Headers and One-Click Semantics — Engineers implement a visible link but omit the standardized POST semantics.
SMTP Reply Codes: What 2xx, 4xx, and 5xx Mean
Learn how to read SMTP reply codes, distinguish successful, temporary, and permanent responses, and choose the right diagnostic action.
What Does Queued Mean in Email?
Queued means an email is waiting for the next step in processing. Learn how to identify the queue, troubleshoot delays, and avoid duplicate sends.
Email Headers Explained: What the Metadata Shows
Learn what email headers contain, how to read them in a useful order, and what routing and authentication fields can—and cannot—prove.