makes no attempt to be compatible with SMTP (#37 on the "You Might Be An
 Anti-Spam Kook If... list at www.rhyolite.com)

 To be precise, the only way to get a message to someone who is using Petmail
 is for the sender to also use Petmail.

  [this puts it firmly in the "what if we could redesign the whole thing from
  scratch" category, and I'll make no attempt to convince you otherwise.
  However, there is still value in actually running it instead of just
  talking about it: it is useful to use PGP on your mail even though some or
  most of your your correspondents don't use it, you may just treat messages
  from one source differently than others]