Some comments:

-# This file implements the minimal SMTP protocol as defined in RFC 821.  It
+# This file implements the minimal SMTP protocol as defined in RFC 5321.  It

Is RFC 5321 completely implemented? Otherwise I would turn this in "as defined in RFC 821 and part of RFC 5321".

> - Implement DATA size limit (32 MiB by default)

I couldn't find any reference to this in RFC 5321. Is it recommended somewhere else maybe?
Also, issue 2518 and issue 1745035 are somewhat related with this specific problem.

> +        ## TODO: implement command help

Since you implemented HELP command in the first place it would be good to do it completely (I guess this means HELP should be able to accept arguments, as in FTP protocol).


Too bad smtpd currently lacks a test suite.
Before going any further with this issue I would recommend to write tests first.
I have a patch for adding a basic test suite for smtpd module I hope I'll be able to submit within this week.