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MailHog SMTP Protocol GoDoc Build Status 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.

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 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 {

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")


The state machine provides hooks to manipulate its behaviour.

See GoDoc 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 GoDoc 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


Copyright ©‎ 2014-2015, Ian Kent (

Released under MIT license, see LICENSE for details.