I'm strongly opposed to MTAs "fixing" malformed messages (other than =
submission servers fixing a small number of known problems caused by =
broken mail clients).
If an MTA does anything at all when it thinks that a message is =
malformed, it should be to bounce it _exactly as it received it =
originally_.
MTAs trying to fix malformed messages, at best, mask problems further =
upstream that should be fixed. At worst, they exacerbate existing =
problems and make such problems harder to diagnose.
Yes, but there are other possibilities (yes, 'fixing' is to be avoided).
But for suitable messages (for some meaning of "suitable") you could send
a bounce back up the return path of the following form:
"Your message is non-compliants for reasons <blah>. Although we have
attempted to deliver it, you need to be aware that it may fail to reach
its intended recipient, and we would suggest that you endeavour to make
future messages compliant with the relevant internet standards <blah list
of standards blah>."
I strongly suspect that the sender's response would be to:
a) find another mail service provider (even at the expense of getting another
email address), and/or
b) stop sending mail to any recipients for whom such bounces happened with any
regularity
...neither of which actually fixes the problem (except for providers that
provider their own user agents)
People these days are so used to dealing with spam and error messages that they
don't understand, that the vast majority of them won't even bother to read such
messages. As soon as they realize it's not intended for them _personally_ ,
the trigger finger reflexively hits the delete key. The user may indeed be
annoyed, but the chance that he'll take useful corrective action is almost zero.
If we want such measures to work, we need to find a way to either
a) send the notifications to the vendors/providers of such user agents, or
b) arrange for them to get triggered only when the user starts using a new user
agent, so he'll immediately associate the user agent with the problem whether
or not he reads the message.
a) can perhaps be accomplished more efficiently in other ways than by annoying
users, e.g. by providing a mail message test suite, advertising it widely, and
inviting vendors to test their products with it.
b) can perhaps be accomplished by having the MSA notice when the user starts
using a new user agent, and only then, notifying the user that his messages
from said new user agent are nonconforming.
That's not to say that I want MSAs to forward thoroughly broken messages; I
don't. But once a message leaves the MSA, I think the opportunity to provide
any kind of useful feedback to the submitter has probably passed.
Keith