erlang-smtp, an SMTP and POP3 server library for Erlang
Back in 2007, I built these simple Erlang modules for generic SMTP and POP3 services. The idea is that the programmer should instantiate a service, providing callbacks for user authentication and for service-specific operations like handling deliveries and scanning and locking mailboxes. Originally, I was planning on providing SMTP-to-AMQP and AMQP-to-POP3 gateways as part of RabbitMQ, but I haven't had the time to seriously pursue this yet (though see rabbitmq-smtp for an experiment in this direction).
The current status of the code is:
SMTP deliveries from Thunderbird and gmail work
POP3 retrieval from Thunderbird works, but isn't very solid, because I haven't implemented the stateful part of mailboxes yet.
The SMTP implementation is somewhat loosely based on RFC 2821. It's what you might generously call minimally conformant (improving this situation is tedious but not difficult). It doesn't address RFC 2822 in any serious way (yet)
The POP3 implementation is based on RFC 1939.
SMTP AUTH is not yet implemented (but is not difficult)
I can't recall the details, but I think I might have skimped on something relating to POP3 UIDL.
Patches, bugfixes, contributions, comments and feedback are all very welcome!
For a different (and much richer!) approach to the same kinds of
problems that this code addresses, see Vagabond's impressive
Things that should probably be done at some point
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