Fantastic, thanks so much! It looks like it's almost there, and definitely something I will check out in more detail. Admittedly, my use case is limited, but as services like Mailgun and Sendgrid are demonstrating -- it is not an insignificant use case. Lamson looks like it's trying to achieve a bit more than that single case.

So a couple of things I noticed that might be missing from Lamson:

(1) Spam blocking auto-updates -- does this tap into Spamhaus, etc.? (2) UTF-8 transcoding? (3) Signature, quote stripping (like Mailgun). (4) DKIM signing? (5) Simple client libraries for sending mail (i.e., I don't want to build a "Lamson application", I just want to talk to a Lamson server on a privileged port using JSON).

Of course, all these concerns might have answers that I'm missing.

A bit more about my (fairly common, I'm guessing) use case: you have a VPS and a MongoDB instance. I want my MTA to take a minimal set of config parameters, say:

-- MongoDB database name -- domain -> MongoDB table mapping -- privileged API sending port

Now I want the MTA to receive mail for me, perform spam blocking, transcode to UTF-8, parse out signature and quotes in a separate field, and dump the whole object to a MongoDB document with the index field = the recipient's email address. I want it to auto-update it's spam blocking rules using whatever external services and internal analysis necessary. I want it to periodically dump some usage statistics into a separate MongoDB table (maybe a circular connection).

Now for sending mail, all I should have to do is connect to the privileged port from a process on the same machine (it's firewalled to the outside world), and submit an HTTP POST request that specifies recipients, message body, attachments, etc. The MTA should accept the request, perform whatever queuing, rate-limiting, and retrying is necessary to send the message.

All failures and diagnostic information are dumped to separate MongoDB tables. The applications deal with the database/storage system directly, to keep the MTA simple.

I'm glossing over a lot, naturally, but I hope that helps.