Accepts mails over SMTP

[link2xt]

Currently mails are read from stdin. Mail should be accepted over SMTP so we can return status line.

There is a Rust implementation of SMTP protocol at https://github.com/stalwartlabs/smtp-proto

It may however be easier to use milter instead of SMTP since we don't need any feature not supported by milter. There are multiple milter protocol implementations:

• Async: https://crates.io/crates/indymilter
• Sync: https://crates.io/crates/rmilter
• libmilter bindings: https://crates.io/crates/milter
  It should also be easy to implement milter without external dependencies.