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.
This is really great. Plus I love the attention to detail in the documentation, and how the demo worked right away. We currently use Mailgun for this but I would love to switch over to using Mailin and host it ourself.
Language detection isn't something I would rely on. Especially since a lot of emails don't contain much text. (Likely even less so with your test message.) Looks like they're using the languagedetect[1] module which also returns a confidence score. This would be a helpful thing to include in the results.
