What about mailing lists that have different sender and
return path addresses?
I know only two kinds of mailing lists, Sender = "Mail From"
and Errors-To = "Mail From", both with "Mail From" list owner.
What do you have in mind ?
Many lists use VERP, which modifies the return path depending on the
recipient of the message, so that the bounce handler can reliably
auto-unsubscribe broken recipients.
And some use it all the time while others use it every so often, making it hard
to predict from one message what the next will look like. (VERP requires
separate message copies, which can be an issue when sending lots of mail to a
bunch of recipients on a single remote host.)
I've also seen other nonstandard fields used to store list sender information,
e.g. x-sender (which of course conflicts with other uses of x-sender as a place
to store authentication information).
I've even seen lists that override the From: field - truly deplorable.
Errors-to is another name for return-path.
There's also some use of an errors-to header field, sometimes coupled to list
expansion, other times not. The claimed advantage of using an errors-to header
field is that it allows for multiple error return addresses while MAIL FROM
does not. And warnings-to as well - claimed advantage there is separation of
warning return from error return.
Actually, it's amazing it all works as well as it does given the amount of
variability out there.
Ned