REFERENCE
Look up the exact thing.
Protocol behavior, record syntax, ports, status codes and the details you need when you're already in the middle of the work.
8BITMIME
The SMTP extension for transporting MIME bodies containing octets beyond 7-bit ASCII.
Abuse Reporting Format
The standardized MIME format used for certain email abuse feedback reports.
Alias vs mailbox
An address that redirects mail is not the same thing as a mailbox that stores it.
ARC
Authenticated Received Chain preserves authentication observations through intermediaries.
Authentication-Results
The standard header used to record message authentication evaluations.
Auto-reply and precedence headers
Common metadata used by automated mail systems and responders.
Auto-Submitted header
A standardized signal for automatically generated email and loop prevention.
Backscatter
Why generating bounces to forged sender addresses can make your mail system part of an abuse problem.
BIMI
Brand Indicators for Message Identification adds a domain-controlled brand indicator on supporting mailbox providers.
Calendar invitations over email
How iCalendar content and scheduling messages travel through normal email.
Catch-all mailboxes
Accepting mail for otherwise nonexistent local parts changes bounce, abuse and directory behavior.
CNAME records in email configurations
Where CNAME is useful and where it cannot replace MX or coexist with other data.
Common enhanced status code families
A field guide to 4.x.x and 5.x.x delivery classifications.
Common ESMTP extensions
How EHLO advertises capabilities beyond original SMTP.
Common MIME multipart types
Alternative, mixed and related represent different relationships between message parts.
Complaint feedback loops
How some mailbox providers return complaint information to eligible senders.
Content-Disposition
How MIME parts indicate inline content or attachments and suggest filenames.
Content-ID and inline images
How MIME body parts can be referenced from HTML without using an external image URL.
Content-Transfer-Encoding
Why MIME uses base64, quoted-printable and related transfer encodings.
Content-Type
MIME media types, boundaries and multipart email.
Core email RFC index
A starting map of the standards behind modern Internet email.
Core email RFC map
A practical map of the standards behind modern Internet email.
DANE for SMTP
DNSSEC-backed TLSA records can authenticate SMTP TLS without relying solely on the public CA model.
Date header
What the Date field represents and why it is not a delivery timestamp.
Delivery receipts vs read receipts
Delivery to a mail system and display by a client are different events.
Delivery Status Notifications
How email systems report delivery failures and status information.
Distribution groups and mailing groups
One recipient address can expand into many delivery recipients.
DKIM
DomainKeys Identified Mail uses cryptographic signatures to authenticate a domain and message content.
DKIM canonicalization
Why harmless-looking message changes sometimes preserve a signature and sometimes break it.
DKIM public key records
What lives under selector._domainkey and how receivers use it.
DKIM selector rotation
A clean selector lifecycle for changing signing keys without invalidating mail in flight.
DKIM-Signature header
Understand selectors, signing domains, canonicalization and signed headers.
DMARC
DMARC connects the visible From domain to SPF or DKIM authentication and adds policy and reporting.
DMARC aggregate reports
How to read aggregate authentication telemetry without mistaking every failure for an attack.
DMARC alignment
How SPF and DKIM identities relate to the visible From domain.
DMARC and subdomains
How organizational-domain policy and the sp tag affect subdomain mail.
DMARC organizational domain
How DMARC determines the policy domain around subdomains.
DMARC reports to another domain
Why external aggregate-report destinations require authorization.
DMARC standards status
RFC 9989 is the current DMARC standards-track specification and obsoletes RFC 7489.
DMARC tags
Reference for common DMARC policy and reporting tags.
DNS blocklists
How IP and domain reputation lists are queried and used.
DNS TTLs for email changes
TTL controls cache lifetime, not how quickly every resolver on Earth will update.
DNSSEC
DNSSEC authenticates DNS data and provides the trust foundation used by DANE.
Does email header order matter?
Some trace and repeated fields have meaningful ordering even though headers are not a simple key-value dictionary.
DSN message format
The machine-readable structure behind many delivery status notifications.
Email blocklists
DNS-based reputation lists are signals used by some receivers, not a universal Internet ban list.
Email journaling
Capturing message copies for organizational retention or compliance workflows.
Email loops
How automatic forwarding and responders can create repeated message circulation.
Email message size limits
Why attachment size and encoded message size are not the same.
Email port guide
What ports 25, 465, 587, 110, 995, 143 and 993 are normally used for.
Email quarantine
What quarantine means operationally and why release permissions matter.
Email service discovery
How clients can discover submission and mailbox services instead of relying only on manual port entry.
Email tracking pixels
What remote-image tracking can observe and why the signal is imperfect.
Enhanced SMTP status codes
The x.y.z code carries more diagnostic structure than the basic three-digit reply alone.
Enhanced status 4.2.2
A temporary mailbox-full condition should normally be retried rather than treated as an invalid address.
Enhanced status 5.1.1
A permanent addressing failure commonly associated with a nonexistent destination mailbox.
Enhanced status 5.7.1
A broad permanent policy/security rejection that requires the accompanying diagnostic text.
Envelope vs header addresses
The SMTP MAIL FROM/RCPT TO envelope versus message From/To headers.
ESMTP extensions
How EHLO advertises SMTP capabilities such as STARTTLS, SIZE and AUTH.
Feedback identifiers in bulk mail
Provider or sender metadata can help correlate complaints and campaigns without exposing recipient data carelessly.
Greylisting
Temporary SMTP deferral as an anti-abuse technique.
Greylisting
Temporary SMTP deferral used by some receivers as an anti-abuse technique.
Hard bounce and soft bounce terminology
Useful operational shorthand that should not replace the actual SMTP status.
How Bcc works
Why a recipient can receive a message without appearing in To or Cc.
HTML email
How HTML fits inside MIME and why web-page assumptions do not transfer cleanly to mail clients.
IMAP and POP3
The traditional protocols used by clients to access stored email.
IMAP IDLE
How an IMAP client can wait for mailbox changes without constant polling.
IMAP message flags
Seen, Answered, Flagged, Deleted and Draft state in IMAP mailboxes.
Implicit MX behavior
What SMTP does when a domain has no MX but does have address records.
Internationalized domains and email
Unicode domain names, Punycode and internationalized email considerations.
Internationalized email messages
How SMTPUTF8 and UTF-8 message headers extend traditional Internet mail.
Internet mail architecture roles
The standard vocabulary for authors, MUAs, MSAs, MTAs, MDAs and related actors.
Internet message format
The basic structure of an email message before MIME adds richer content.
IPv6 and email
AAAA records and IPv6 sending require the same operational discipline as IPv4, plus working reverse DNS.
List-Unsubscribe headers
Headers used to expose mailing-list unsubscribe mechanisms.
Mailbox forwarding
Automatic forwarding is useful, but it changes routing, data exposure and authentication behavior.
Mailbox rules
Server-side rules can automate mail handling—and hide attacker activity.
Mailbox-provider postmaster tools
What provider dashboards can tell senders and what they cannot.
Mailing list headers
Standard List-* fields used by mailing-list software.
Message-ID
A message identifier useful for correlation but not a cryptographic identity.
MIME
The structure that lets email carry HTML, attachments, multiple body alternatives and non-ASCII content.
MTA-STS
A policy mechanism for requiring authenticated TLS when other mail servers deliver to your MX hosts.
MTA-STS policy file syntax
The HTTPS policy tells supporting senders which MX hosts are valid and whether enforcement is active.
MX preference values
Lower MX numbers are preferred, but preference is not a traffic-weighting percentage.
MX priority and failover
How senders choose between multiple MX records and what backup MX really means.
MX records
Syntax and behavior of DNS mail exchanger records.
Non-ASCII text in email headers
How older encoded-word syntax and internationalized headers handle characters beyond ASCII.
Null MX
How a domain explicitly states that it accepts no email.
Null reverse-path
Why bounces use MAIL FROM:<> and why responders should not bounce a bounce.
OAuth and modern mail authentication
Token-based authorization for mail clients and applications.
OAuth with SMTP, IMAP and POP
How token-based authorization can replace long-lived mailbox passwords.
One-click unsubscribe
The RFC 8058 headers used by mailbox providers for automated unsubscribe actions.
Open SMTP relays
Why unrestricted third-party relay is an abuse and reputation disaster.
OpenPGP email
Message-level signing and encryption using OpenPGP/MIME.
Per-recipient SMTP responses after DATA
Why multi-recipient messages can be awkward when a server wants different final outcomes per recipient.
Plus addressing
Using a tag in the local part of an address for routing or organization.
Postmaster and abuse mailboxes
Operational role addresses used around Internet mail domains.
Preheaders and preview text
The text mailbox clients may show beside or beneath a message subject.
PTR and forward-confirmed reverse DNS
Why outbound mail operators commonly align reverse and forward DNS for sending hosts.
PTR and reverse DNS
How reverse DNS is used around outbound email infrastructure.
Quoted-printable and Base64
Common MIME content-transfer encodings.
Read receipts and disposition notifications
Why a requested read receipt is not reliable proof a person read a message.
Received header
Reference for SMTP Received trace fields.
Received-SPF vs Authentication-Results
Two ways authentication observations may appear in headers.
Reply-To header
How Reply-To changes the destination for replies without changing From.
REQUIRETLS
An SMTP extension for requesting TLS protection across message transport.
Retention is not backup
Why compliance retention, mailbox recovery and disaster recovery are different controls.
Return-Path and envelope sender
How SMTP envelope identity differs from the visible From header.
Role-based email addresses
Addresses such as postmaster and abuse represent functions rather than individual people.
S/MIME
Certificate-based signing and encryption for message content.
SASL in email protocols
The authentication framework used by SMTP AUTH and other application protocols.
Sender Rewriting Scheme
How forwarders can rewrite the envelope sender to address SPF forwarding failures.
Shared mailboxes and delegated access
Separate mailbox identity from the people authorized to read or send as it.
Sieve email filtering
A standardized language for server-side mail filtering.
SMTP
The protocol that moves email between clients, submission services and mail servers.
SMTP AUTH
Authentication on message submission services.
SMTP CHUNKING and BDAT
An alternative to DATA for transferring message content in chunks.
SMTP command quick reference
The common commands you will see in a normal SMTP transaction.
SMTP DATA and dot-stuffing
How SMTP marks the end of message data without truncating body lines that begin with a dot.
SMTP DSN extension
Requesting delivery status notification behavior during an SMTP transaction.
SMTP EHLO identity
The hostname a sending SMTP client presents is part of the transaction and useful operational evidence.
SMTP message submission
Why application/client submission is separated from server-to-server SMTP relay.
SMTP PIPELINING
An ESMTP extension that reduces round trips by allowing commands to be sent without waiting for every reply.
SMTP ports
What ports 25, 465 and 587 are normally used for.
SMTP queue lifetime
How long a sender retries is an operational policy bounded by SMTP requirements and implementation choices.
SMTP queues and retry behavior
How MTAs handle temporary delivery failures.
SMTP rate limiting
Throttling connections or messages without confusing temporary controls with permanent rejection.
SMTP Received trace fields
What a mail server records when it adds a Received header.
SMTP retry backoff
Why temporary failures should lead to spaced retries rather than a tight retry loop.
SMTP SIZE extension
How servers advertise message-size capability and clients can declare a message size before DATA.
SMTP status codes
Read SMTP replies and enhanced status codes without guessing from a bounce subject line.
SMTP tarpitting
Deliberately slowing suspicious SMTP sessions to increase the cost of abuse.
SMTP timeouts and slow sessions
Why mail servers cannot wait forever for the next command or response.
SMTP TLS certificates
Names, trust and validity checks in SMTP transport and submission.
SMTPUTF8
Internationalized mailbox and header support in SMTP.
SPF
Sender Policy Framework authorizes SMTP sources for a domain.
SPF DNS lookup limit
Why a syntactically valid SPF record can still return permerror.
SPF flattening
Why replacing includes with IP addresses trades DNS lookups for maintenance risk.
SPF mechanisms and qualifiers
Reference for include, ip4, ip6, a, mx, exists, all and qualifiers.
SPF result meanings
Pass, fail, softfail, neutral, none, temperror and permerror.
Standard operational mailboxes
Common role addresses defined for Internet operations.
STARTTLS
Opportunistic TLS negotiation for SMTP, IMAP and related protocols.
TLS versions and cipher policy for email
Transport security requires interoperable cryptography as well as a valid certificate.
TLS-RPT
Aggregate reporting for failures involving SMTP TLS and transport security policy.
TLS-RPT DNS record
The DNS policy that tells participating senders where to send SMTP TLS reports.
TLSA records for SMTP DANE
How DNSSEC-protected TLSA records can authenticate SMTP TLS endpoints.
TNEF and winmail.dat
Why some Outlook-originated messages expose a winmail.dat attachment.
TXT and CNAME records in email
Common DNS record patterns used by hosted email services.
TXT records in email
Why so many email controls use DNS TXT and why record boundaries still matter.
Variable envelope return paths
How bulk senders can encode recipient or campaign information into bounce addresses.
Why MX targets should not be CNAME aliases
Mail exchanger targets are expected to resolve directly to address records.