session_connect is called when a client first connects, before any messages are
accepted.
`Accept greeting initiates a session state and sends the given greeting.
`Disconnect maybe_reply terminates the connection sending the given
reply. disconnect is NOT called.
session_helo is called in response to initial handshakes (i.e. HELO or EHLO).
`Continue will update the session state.
`Deny reply will send the given reply but leave the session open.
`Disconnect maybe_reply will send the given reply and close the session
(disconnect is not called)
process_sender is called in the event of a "MAIL FROM" SMTP command.
`Continue creates an envelope that is filled in by subsequent commands.
`Reject reply will abort this message.
process_recipient is called in the event of a "RCPT TO" SMTP command.
`Continue augments the envelope in the pipeline and passes it on to the next phase.
`Reject reply pass through an unmodified envelope to the next phase, omitting
this address from the recipients list.
Called after the message body has been received:
`Consume ok_msg will discard the message and return a successful response to the
client. It should be used if this plugin has processed the message and does not
intend to relay the message (or a response).
`Reject reply will discard the message and return an error reply to the client. It
should be used when the plugin does not wish to process this message.
`Send envelopes_to_relay will spool the given envelopes for further sending and
give a successful response, once these messages have been safely spooled.
`Quarantine (envelopes_to_quarantine, reply, reason) will save the given messages
to a directory for manual inspection and pass through the given reply to the
client. reason is used only internally to tell us what check failed.