(Please don't send me duplicates, Chris.)
There are two cases where MUAs need to cooperate with subaddresses:
1) MUAs which don't allow from to be edited otherwise.
This is a purely local configuration problem.
2) MUAs which validate local addresses against a white pages or authdb
A service that claims to check mail addresses obviously has to
understand subaddresses. This is a purely local configuration problem.
1) List server restricts posting to subscribers.
You have no right to demand that list owners accept your mail.
There are several cases where a final delivery agent needs to cooperate
All of those cases are purely local configuration problems.
I suppose next we're going to see a document explaining how sendmail
interacts with procmail, and requiring that all MTAs and LDAs work the
Set up a new mailing list in a single command. http://pobox.com/~djb/ezmlm.html