Glossary
Authoritative, concise definitions of the core email, authentication, deliverability, and infrastructure terms, each linking to the in-depth guide.
24 guides and growing
Why this matters
Email has a vocabulary of its own, and the terms are not decoration. SPF, DMARC, SCL, PTR, warm-up: each names a specific mechanism that decides whether your mail reaches the inbox. These are the canonical definitions, each linking to the guide that explains it in depth.
What you’ll be able to do
The guides
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.
Authentication-Results Header: Reading Receiver Assertions
Authentication-Results Header: Reading Receiver Assertions — Analysts need to read receiver-side SPF, DKIM, and DMARC results.
BIMI (Brand Indicators for Message Identification)
BIMI displays your verified brand logo next to authenticated email. It requires an enforced DMARC policy first.
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.
Click-Through Rate vs. Click-to-Open Rate
Click-Through Rate vs. Click-to-Open Rate — Teams compare denominators incorrectly across reports.
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.
DNS Records for Email
DNS records (MX, PTR, SPF, DKIM, DMARC) are where every deliverability check begins. A definition, with the full reference linked.
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.
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.
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.
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.
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.
Inbox Placement
Inbox placement is where an accepted message lands: inbox, Promotions, or spam. A definition, distinct from deliverability, with the guide linked.
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.
MX Record
An MX record tells other servers where to deliver mail for your domain. It governs where replies to your cold email land.
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.
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.
Sender Reputation
Sender reputation is the trust score receivers assign your domain and IP. A definition, with the full guide linked.
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.
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.
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.
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.