a tiny UDP-based Mail Transport Agent written in pd can be found here: http://puredata.info/Members/zmoelnig/pd-mta/view
dependencies: pd>=0.39 zexy>=2.0
features: only MTA i know that speaks UDP almost(!) SMTP compliant comes with tiny Mail User Agent that can be used with the MTA (well, its rather a "sendmail" implementation than an MUA)
issues: only speaks UDP until now SMTP is wrapped into FUDI (which is not SMTP compliant)
i wrote it within a few hours because i needed it for a performance (transmitting an email via semaphore flags). i was amazed how fast an SMTP server can be done within pd (after all it is a rather complex software); if the license texts included into the patch are omitted, the patch is less than half the size of the GnuGPL license file.
mfg.adsr IOhannes