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Email glossary
Every technical term used across the SMTPedia reference, defined clearly and cross-linked to its full deep-dive. From SPF to X-Headers, grouped by concept.
Email has a vocabulary problem. The same word means different things depending on whether you are a marketer, a developer, or a deliverability engineer. "Bounce" can mean anything from a hard address failure to a policy rejection to a full pipeline failure. "Reputation" spans domain reputation, IP reputation, and sender reputation, which are three related but distinct signals.
This glossary covers the terms that matter for email infrastructure and deliverability. Each entry starts with a plain-language definition, then links to the practitioner deep-dive where the full context lives. Grouped by concept rather than alphabet so related terms cluster naturally: authentication terms together, DNS terms together, delivery terms together.
If you are onboarding onto an email team, working through a deliverability audit, or just want to make sure everyone in a meeting means the same thing by "warm-up", start here.
Authentication terms
SPF, DKIM, DMARC, ARC, BIMI: the cryptographic and DNS-based sender identity mechanisms.
Read guides →DNS terms
MX, TXT, PTR, A/AAAA, CNAME, TLSA: the DNS surface that email operates on.
Read guides →Delivery terms
Deliverability, inbox placement, bounce, complaint rate, engagement: the outcome vocabulary.
Read guides →Protocol terms
SMTP, IMAP, POP3, ESMTP, STARTTLS: the protocols that move messages between servers and clients.
Read guides →Bounce and error terms
Hard bounce, soft bounce, DSN, NDR, RFC 3463 codes: the outcome side of failed delivery.
Read guides →Reputation terms
Sender Score, domain reputation, IP reputation, warm-up, complaint rate: how receivers rate you.
Read guides →Learn SPF, DKIM, and DMARC first: they are the authentication foundation and every other topic references them. Then MX (how mail is routed) and PTR (why receivers trust your outbound IPs). Then bounce handling (how to react when things fail). This gives you the vocabulary to read production incidents and pick up specialized topics as needed.
Deliverability is whether your message was accepted by the receiver (technically delivered, not bounced). Inbox placement is where the accepted message ended up: inbox, promotions tab, spam folder, or dropped silently after acceptance. A message can have 100 percent deliverability and near-zero inbox placement if the receiver accepts everything and filters aggressively. Modern email metrics track both.
Yes, and increasingly enforced. Gmail and Yahoo bulk sender rules (2024) require SPF and DKIM to pass, DMARC to be published, and DMARC alignment for high-volume senders. Microsoft has moved in the same direction. Missing any of the three now produces measurable delivery drops. The old "just SPF is enough" advice is dead.
In modern usage they overlap, but the distinction matters. A bounce is a delivery failure notice returned to the sender (usually as a DSN following RFC 3463/3464). A rejection is the SMTP-level response from the receiver refusing the message during the transaction (5xx code). Every rejection produces a bounce; not every bounce comes from a clean rejection (silent drops and post-acceptance filtering exist too).
FCrDNS stands for Forward-Confirmed Reverse DNS: the sending IP has a PTR record pointing to a hostname, and that hostname resolves via A/AAAA back to the same IP. This closes the identity loop and prevents PTR spoofing. Gmail and Yahoo have enforced FCrDNS since the 2024 bulk sender rules; missing or broken FCrDNS produces 550 5.7.25 rejections that no other fix will resolve.
IMAP keeps mail on the server and syncs state across devices; read a message on your phone, it shows as read on your laptop. POP3 downloads mail to the local device and (by default) deletes it from the server. IMAP is the modern default for multi-device use; POP3 only makes sense for single-device, offline-first workflows with tight server storage. Both are retrieval protocols; SMTP is the sending protocol.