🚀 go-pugleaf

I modified the delivery part of Rand MH to do something similar, although
I used "." instead of "=", e.g. "greep.foomail" means user greep, mailbox
foomail.  What then happens is that if the recipient has specified his own
program to handle his incoming mail (this was already part of MH), one of
the arguments that is passed to that program is the mailbox, in this case
"foomail".  The program can do whatever it wants with the message.  If the
address consists of a user name with no mailbox, then the argument defaults
to "inbox".  If the user does not have his own program to handle incoming
mail, then the part after the period is ignored and the mail all goes in
one place.

This is how we handle bboards.  For example, "dsn-WorkS" is aliased to
"news.works".  There is a fake user called "news" which has a shell file
to handle incoming mail.  The shell program makes sure that the mailbox
specified exists and then puts the message in it.  If the mailbox isn't
valid, the program returns a non-zero return code which tells the mailer
to notify the sender of the error by sending back a message, which will
include any error messages that were output by the program.