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MailHog SMTP Protocol

github.com/mailhog/smtp implements an SMTP server state machine.

It attempts to encapsulate as much of the SMTP protocol (plus its extensions) as possible without compromising configurability or requiring specific backend implementations.

ESMTP server implementing RFC5321
Support for:
- AUTH RFC4954
- PIPELINING RFC2920
- STARTTLS RFC3207

proto := NewProtocol()
reply := proto.Start()
reply = proto.ProcessCommand("EHLO localhost")
// ...

See MailHog-Server and MailHog-MTA for example implementations.

Commands and replies

Interaction with the state machine is via:

the Parse function
the ProcessCommand and ProcessData functions

You can mix the use of all three functions as necessary.

Parse

Parse should be used on a raw text stream. It looks for an end of line (\r\n), and if found, processes a single command. Any unprocessed data is returned.

If any unprocessed data is returned, Parse should be called again to process then next command.

text := "EHLO localhost\r\nMAIL FROM:<test>\r\nDATA\r\nTest\r\n.\r\n"

var reply *smtp.Reply
for {
  text, reply = proto.Parse(text)
  if len(text) == 0 {
    break
  }
}

ProcessCommand and ProcessData

ProcessCommand should be used for an already parsed command (i.e., a complete SMTP "line" excluding the line ending).

ProcessData should be used if the protocol is in DATA state.

reply = proto.ProcessCommand("EHLO localhost")
reply = proto.ProcessCommand("MAIL FROM:<test>")
reply = proto.ProcessCommand("DATA")
reply = proto.ProcessData("Test\r\n.\r\n")

Hooks

The state machine provides hooks to manipulate its behaviour.

See for more information.

Hook	Description
LogHandler	Called for every log message
MessageReceivedHandler	Called for each message received
ValidateSenderHandler	Called after MAIL FROM
ValidateRecipientHandler	Called after RCPT TO
ValidateAuthenticationHandler	Called after AUTH
SMTPVerbFilter	Called for every SMTP command processed
TLSHandler	Callback mashup called after STARTTLS
GetAuthenticationMechanismsHandler	Called for each EHLO command

Behaviour flags

The state machine also exports variables to control its behaviour:

See for more information.

Variable	Description
RejectBrokenRCPTSyntax	Reject non-conforming RCPT syntax
RejectBrokenMAILSyntax	Reject non-conforming MAIL syntax
RequireTLS	Require STARTTLS before other commands
MaximumRecipients	Maximum recipients per message
MaximumLineLength	Maximum length of SMTP line

Licence

Released under MIT license, see LICENSE for details.