with few exceptions, the mail transport system should not be
doing mail filtering. the mail transport's job is to deliver
mail, not to make filtering decisions on behalf of the user.
if recipients want to filter their own mail, that's their
business, but then it's a UA-level thing.
I disagree. Filtering is something that *is* done at the delivery end of the
transport cycle. A "UA" relies on delivery and transport agents to make a
number of decisions. If it was merely a UA activity, we wouldn't have
vacation, procmail, .forward, IMAP BB+ -style delivery, auto-bouncing, or
any current filtering activities done completely without the participation
of a UA.
vacation and procmail *are* UAs, as are any programs to which mail is
forwarded for the purpose of filtering mail.
I can understand why a ubiquitous filtering language would be useful.
I can also understand how it could cause a great deal of harm, by making
the behavior of the mail transport system less predictable.
Defining only the language also ignores the security issues. How
are users going to specify such filtering without some means of
authenticating themselves to the filter?