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4. The SMTP Specifications

This section contains the detailed specifications for SMTP commands, replies, sequencing, trace information, and implementation issues.

4.1. SMTP Commands​

4.1.1. Command Semantics and Syntax​

SMTP commands are 4-letter (some extended to 8) case-insensitive ASCII words optionally followed by parameters.

Extended HELLO (EHLO) or HELLO (HELO)​

Syntax: EHLO domain or HELO domain

The EHLO command initiates an SMTP session and requests service extensions. HELO is the fallback for servers not supporting extensions.

Example:

C: EHLO client.example.com
S: 250-server.example.com
S: 250-SIZE 52428800
S: 250-8BITMIME
S: 250 STARTTLS

MAIL (MAIL FROM)​

Syntax: MAIL FROM:<reverse-path> [parameters]

Initiates a mail transaction by specifying the sender.

Example:

C: MAIL FROM:``<[email protected]>``
S: 250 Ok

Parameters: May include SIZE, BODY, AUTH, etc.

RECIPIENT (RCPT TO)​

Syntax: RCPT TO:<forward-path> [parameters]

Identifies one recipient of the mail.

Example:

C: RCPT TO:``<[email protected]>``
S: 250 Ok

DATA​

Syntax: DATA

Indicates start of message content. Content ends with <CRLF>.<CRLF>.

Example:

C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: [email protected]
C: To: [email protected]
C: Subject: Test
C: 
C: Message body here.
C: .
S: 250 Ok: queued as 12345

RESET (RSET)​

Syntax: RSET

Aborts current mail transaction and resets buffers.

Example:

C: RSET
S: 250 Ok

VERIFY (VRFY)​

Syntax: VRFY string

Verifies that a mailbox exists.

Example:

C: VRFY Jones
S: 250 Fred Jones ``<[email protected]>``

Note: Many servers disable this for security reasons.

EXPAND (EXPN)​

Syntax: EXPN string

Expands a mailing list.

Example:

C: EXPN staff
S: 250-Alice ``<[email protected]>``
S: 250 Bob ``<[email protected]>``

HELP​

Syntax: HELP [command]

Provides help information.

Example:

C: HELP
S: 214-This server supports the following commands:
S: 214 EHLO MAIL RCPT DATA RSET VRFY HELP QUIT

NO OPERATION (NOOP)​

Syntax: NOOP

Does nothing, used to keep connection alive.

Example:

C: NOOP
S: 250 Ok

QUIT​

Syntax: QUIT

Terminates the SMTP session.

Example:

C: QUIT
S: 221 server.example.com closing connection

4.1.2. Command Argument Syntax​

Commands follow strict syntax rules defined in ABNF (RFC 5234).

Mailbox Format: local-part@domain

  • Local-part: up to 64 characters
  • Domain: fully qualified domain name or address literal

Address Literals:

  • IPv4: [192.0.2.1]
  • IPv6: [IPv6:2001:db8::1]

4.1.3. Address Literals​

Address literals are domains enclosed in brackets:

  • [192.0.2.1] - IPv4 address
  • [IPv6:2001:db8::1] - IPv6 address

4.1.4. Order of Commands​

Valid command sequences:

  1. Connection: Server sends 220
  2. Greeting: Client sends EHLO/HELO
  3. Transaction: MAIL → RCPT (one or more) → DATA
  4. Termination: QUIT

Invalid sequences result in 503 "Bad sequence of commands" error.

4.1.5. Private-Use Commands​

Commands starting with X are reserved for private use and experimentation.

4.2. SMTP Replies​

4.2.1. Reply Code Severities and Theory​

Reply codes are 3-digit numbers:

  • 2yz: Positive completion
  • 3yz: Positive intermediate
  • 4yz: Transient negative (retry possible)
  • 5yz: Permanent negative (do not retry)

Second digit indicates category:

  • x0z: Syntax
  • x1z: Information
  • x2z: Connections
  • x5z: Mail system

4.2.2. Reply Codes by Function Groups​

Connection Establishment:

  • 220: Service ready
  • 421: Service not available

Client Initiation:

  • 250: OK
  • 500: Syntax error
  • 502: Command not implemented

Mail Transactions:

  • 250: OK
  • 354: Start mail input
  • 450/550: Mailbox unavailable
  • 451/551: Error in processing
  • 452/552: Insufficient storage
  • 553: Mailbox name invalid
  • 554: Transaction failed

4.2.3. Reply Codes in Numeric Order​

  • 211: System status
  • 214: Help message
  • 220: Service ready
  • 221: Closing connection
  • 250: OK
  • 251: User not local; will forward
  • 252: Cannot VRFY user, but will accept
  • 354: Start mail input
  • 421: Service not available
  • 450: Mailbox unavailable
  • 451: Local error
  • 452: Insufficient storage
  • 455: Server unable to accommodate parameters
  • 500: Syntax error
  • 501: Syntax error in parameters
  • 502: Command not implemented
  • 503: Bad sequence of commands
  • 504: Command parameter not implemented
  • 550: Mailbox unavailable
  • 551: User not local
  • 552: Storage allocation exceeded
  • 553: Mailbox name not allowed
  • 554: Transaction failed
  • 555: Parameters not recognized

4.2.4. Reply Code 502​

502 indicates command not implemented. Clients should not retry unimplemented commands.

4.2.5. Reply Codes after DATA and the Subsequent <CRLF>.<CRLF>​

After DATA completion:

  • 250: Message accepted
  • 450/550: Mailbox unavailable
  • 451/551: Processing error
  • 452/552: Insufficient storage
  • 554: Transaction failed

4.3. Sequencing of Commands and Replies​

4.3.1. Sequencing Overview​

SMTP is lock-step: client sends command, waits for reply, then sends next command. Exception: PIPELINING extension allows batching.

4.3.2. Command-Reply Sequences​

Minimum Implementation:

S: 220
C: EHLO
S: 250
C: MAIL FROM
S: 250
C: RCPT TO
S: 250
C: DATA
S: 354
C: (message data)
C: .
S: 250
C: QUIT
S: 221

4.4. Trace Information​

Each SMTP server adds a Received header field when accepting a message:

Format:

Received: from sender.example.com (host.example.com [192.0.2.1])
  by receiver.example.com (Postfix) with ESMTP id 12345
  for ``<[email protected]>``; Mon, 24 Dec 2024 10:00:00 +0000 (UTC)

Components:

  • from: Sending host identification
  • by: Receiving host
  • with: Protocol used (SMTP, ESMTP, etc.)
  • for: Recipient address
  • Timestamp

4.5. Additional Implementation Issues​

4.5.1. Minimum Implementation​

Fully-functional SMTP must support:

  • Commands: EHLO, MAIL, RCPT, DATA, RSET, NOOP, QUIT
  • Reply codes: All standard codes
  • Queueing and retry logic
  • Proper error handling

4.5.2. Transparency​

Servers must handle dot-stuffing: lines beginning with . must be escaped as .. during DATA transmission.

4.5.3. Sizes and Timeouts​

4.5.3.1. Size Limits and Minimums​

Minimum Requirements:

  • Local-part: 64 octets
  • Domain: 255 octets
  • Path: 256 octets
  • Command line: 512 octets
  • Reply line: 512 octets
  • Text line: 1000 octets
  • Recipients buffer: 100 recipients

4.5.3.2. Timeouts​

Client Timeouts:

  • Initial 220 message: 5 minutes
  • MAIL command: 5 minutes
  • RCPT command: 5 minutes
  • DATA initiation: 2 minutes
  • Data block: 3 minutes
  • DATA termination: 10 minutes

Server Timeouts:

  • Waiting for command: 5 minutes
  • Waiting for DATA: 10 minutes

4.5.4. Retry Strategies​

On transient failures (4yz codes):

  • Retry at least every 30 minutes
  • Continue trying for at least 4-5 days
  • Send warnings to sender at intervals
  • Generate bounce after final failure

4.5.5. Messages with a Null Reverse-Path​

Bounce messages use MAIL FROM:<> (null reverse-path) to prevent mail loops. Servers MUST accept null reverse-path but MUST NOT generate bounces for them.