vacation and procmail *are* UAs, as are any programs to which mail is
forwarded for the purpose of filtering mail.
They aren't for a Windows 95 user running IMAP4 to a blackbox server
that doesn't use UNIX accounts to provide IMAP account authentication.
If the filtering language is to be transmitted over the 'net between
a user and his mail service, then it's part of an on-the-wire protocol.
the filtering language might be thought of as a separate piece, but it's
going to need some sort of transfer protocol with authentication to be
useful in this scenario.
If the filtering language is not to be transmitted over the 'net,
then arguably it's system-private matter and not appropriate for
standardization. (I don't entirely agree with this, but one
of the traditional rules of thumb is that system-private matters
don't belong in IETF)
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.
You'll get this with any filtering tool. Procmail is a very dangerous
thing in the wrong hands. The fact that people use these dangerous
tools regardless tells me there is a requirement for the functionality.
Mail filtering is certainly useful. My concerns are about whether
standarization of a mail filtering language is an appropriate activity
for IETF (given its tranditional scope and limited resources) and if so,
how to legitimize mail filtering without adversely impacting the mail
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?
The point of SIEVE is to define the filtering language. How or where it
gets stored is up to consenting clients and servers, although it's
anticipated that ACAP will be the storage medium of choice.
Users will need a method to specify filtering to their ISPs. If this
method is not defined as part of the standard, different clients and
different ISPs will support different ways of specifying the filter,
and those users will have interoperability problems.
I wonder if ISPs would really be willing to support ACAP as their
storage medium of choice for mail filtering.