This project looks like a wrapper over andris9/simplesmtp, and should be advertised as such instead of a smtp implementation. Check out https://github.com/baudehlo/Haraka for a mature smtp project instead of a wrapper.
I'll second the vote for Haraka. Good support via IRC, and a very strong, accessible plugin architecture. I like the basic mechanism Mailin is providing (convert e-mail to a semi-structured doc) - I just don't know their smtp wrapper's reliability. It's tempting to just build a Haraka plugin to provide a similar output (and feed it to Elasticsearch for logging/searching later on).
Reading through the source it looks like it spawns a child_process per-message to for checking both SPF and DKIM via a python script... That feels inefficient, but I don't know if there's a better node-based solution.
The main objective of mailin is providing the service email-to-webhook with a very easy setup.
You're right, for the spawned child processes, the correct way to do it while still using python would be to have some running daemons to connect to.
