Mails are allowed to have multiple addresses in the from header as
long as they also have a sender header. However, in basic testing, I
have yet to find a MUA that even displays multiple entries for from.
Has anyone seen such a user agent?
Alpine lets you create them and displays them reasonably.
I'm a bit vague on what their intended use is/was.
Any sort of message where a paper version would have two signatures.
It's always been there, rarely been used other than to prove that it's
possible, but there are a few people who passionately believe that
it's important.
Given how little it's used, I'd think it should have been deprecated
or deleted in 5322 in the interest of better interop, but that
apparently wasn't even on the table.
R's,
John