The most significant item that needs to be called out is the issue of
tunneling the PRIORITY value through non-conforming MTAs by turning it
into a message header field (MT-Priority) and then back again. This
is a problematic technique, but is an important capability for those
who need and intend to implement this extension. It creates a trust
issue, wherein a message containing MT-Priority can be originated with
a Message Submission Agent that does not know about this extension,
and when the message hits a Message Transfer Agent that does support
this, the header field will be turned back into a valid PRIORITY
value, on the unwarranted assumption that it was authorized.
Intermediate MTAs have no way to distinguish this situation from one
where the field was tunneled legitimately.
There may not be substantial experience with doing this sort of thing in SMTP
relays, but there's plenty of experience doing it in gateways to other mail
environments, e.g., X.400 and many of the old LAN email systems. In fact one of
the more common fields that has been mapped this way is message transfer
priority, so there is considerably experience with fields having more or less
the same semantics as what is being proposed here.
I am unaware of any cases where this was abused, probably because increased
transfer priority doesn't buy you all that much in most cases. Related
but more user-visible features, e.g., importance (in the X.400 sense)
have been known to be abused, however.
That said, I think an important omission in this document is that it
only allows MSA's to change message priorities to conform to site policy.
MTAs should also be allowed to do this.
Another issue is the silly prohibition against using Priority: and other header
fields to set priority levels. What if is existing site policy is in fact to
use those fields to determine message priority?
The counter-argument is that the use case for this specification
involves out-of-band trust relationships, and that such situations
will be known and dealt with. I believe that limits the usability of
those features on the open Internet, with other use cases.
Not if MTAs aren't also allowed to enforce site policy.
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