LMTP support in cmail-smtpd

[cbdevnet]

The LMTP protocol (RFC 2033) differs from ESMTP in only a few ways

• Port used
• EHLO command replaced with LHLO
• One delivery response per recipient

Implementing this protocol would allow cooperation with a wide range of other software.

This could be done in cmail-smtpd by

• Allowing the LHLO command in the state machine and setting the proper protocol in client_data->current_mail.protocol -> https://github.com/cmail-mta/cmail/blob/master/cmail-smtpd/smtpstatemachine.c#L5
• Responding with one recipient state for each handled recipient (this might require modifications of core handling) -> https://github.com/cmail-mta/cmail/blob/master/cmail-smtpd/smtpstatemachine.c#L557 and call tree
• Optionally, if allowing LMTP on all ports (as the current implementation handles SMTP/ESMTP) is not desirable
  • Extending the listener structure with a field indicating supported protocols -> https://github.com/cmail-mta/cmail/blob/master/cmail-smtpd/smtpd.h#L83
  • Extending the configuration parser to force a port to specific protocols -> https://github.com/cmail-mta/cmail/blob/master/cmail-smtpd/config.c#L33
  • Enforcing that choice in the state machine (by rejecting commands not allowed for the forced protocol) -> https://github.com/cmail-mta/cmail/blob/master/cmail-smtpd/smtpd.h#L97