Glossary

Terms this manual uses without explaining them again.

Addressing#

Address — In Corsair, a mailbox credential or a routing entry, of one of four kinds: standard, catchall, alias, or group. Distinct from a user.

Alias — An address with no mailbox and no password that forwards to exactly one destination.

Catch-all — A mailbox that also receives anything in the domain matching nothing else. One per domain.

Group — An address with no mailbox that fans out to several destinations.

Local part — The bit before the @.

Sub-addressing — you+tag@example.com routing to you@example.com. Also called plus-addressing. Works with no configuration.

User — A control-panel login that owns domains. Never a mailbox credential.

Authentication#

DKIM — DomainKeys Identified Mail. A cryptographic signature over the message, verified against a public key in DNS. Survives forwarding.

DMARC — Tells receivers what to do when a message claiming to be from your domain passes neither an aligned SPF nor an aligned DKIM check. Passes if either does.

Alignment — Whether the domain that passed SPF or DKIM matches the domain in the From: header. An unaligned pass does not satisfy DMARC.

Selector — Names one DKIM key, so several can coexist. Corsair creates three per domain; the receiver reads which was used from the signature's s=.

SPF — Sender Policy Framework. A DNS record listing which servers may send as your domain. Broken by forwarding.

SRS — Sender Rewriting Scheme. Rewrites the envelope sender on forwarded mail so it passes the next hop's SPF check. Corsair does this automatically; the HMAC in the rewritten address is what stops it being an open relay.

Delivery#

Backscatter — A bounce sent to a forged sender. Accepting mail and bouncing it later makes you a source of spam for whoever was forged. Corsair rejects during the SMTP transaction instead.

Bounce — A permanent delivery failure returned to the sender.

Deferral — A temporary failure (4xx). The sender retries.

Envelope — The MAIL FROM and RCPT TO of the SMTP transaction, as opposed to the From: and To: headers. They need not match, and filters should usually match on the envelope.

Greylisting — Temporarily refusing a first delivery attempt from an unknown sender, on the theory that spam software does not retry. Normal; needs no action.

MX — The DNS record naming where a domain's mail is delivered.

PTR — The reverse DNS record mapping an IP back to a hostname. Must match CORSAIR_HOSTNAME, or large providers reject on connect.

Relay / smarthost — An upstream server you hand outbound mail to instead of delivering it yourself. The answer when port 25 is blocked.

Submission — Authenticated sending by your own users, on 587 or 465. Distinct from the MX role on port 25.

Protocols#

IMAP — The protocol clients use to read mail that stays on the server. Folders, flags, and multiple devices.

JMAP — A modern JSON-over-HTTP alternative to IMAP, with batched calls and incremental sync.

POP3 — Downloads and usually deletes. No folders, no shared state.

SMTP — The protocol that moves mail between servers, and that clients use to send.

STARTTLS — Upgrading a plaintext connection to TLS mid-session. Distinct from implicit TLS, which is encrypted from the first byte.

MTA-STS — A policy telling senders to require TLS when delivering to you.

Storage#

Flag — Per-message state: \Seen, \Answered, \Flagged, \Deleted, \Draft, plus any keyword a client sets.

Expunge — Permanently removing messages marked \Deleted. Emitted highest-sequence-first, because IMAP renumbers after every one.

Modseq — A monotonically increasing number per change, used for incremental sync.

Sequence number — A message's position in the folder, which renumbers when anything is expunged. Not stable; use a UID.

Special use — An attribute marking a folder's role (\Sent, \Junk) so clients file things correctly without configuration.

UID — A message's permanent identifier within a folder. Never reused, never changed. A duplicate UID is the one thing an IMAP client never recovers from.

UIDVALIDITY — A folder generation number. If it changes, a client throws away its cache and starts over.

Corsair specifics#

Entitlement — An account's plan plus its live subscription. Determines storage, daily limits, and which features are available.

Fallback domain — A domain that unmatched recipients are sent on to, followed exactly once.

Instance owner — The first user created. users.is_owner, guarded by a partial unique index.

Plate — Not a Corsair term. It is the bordered panel this documentation site is built out of.

Sieve — The standard mail filtering language, RFC 5228. No loops, no recursion, no network calls — which is why it is safe to run a user's script on the delivery path.

Tombstone — A record that a message was expunged, kept until the retention sweep so a client that was offline can be told what disappeared.

Unmetered — An instance with no plans configured. Every feature on, no caps.

Reply codes#

2xx — Success.

4xx — Temporary. The sender will retry, often for days.

5xx — Permanent. The message bounces.

Enhanced status code — The second, dotted code (5.1.1) carrying more detail than the three-digit reply. See SMTP error lookup.