Awesome

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 gen_smtp.

Things that should probably be done at some point

Follow djb's advice on reading and writing 822/822bis messages and general email infrastructure

Licensing

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.