Suppose a mail-based service needs to send an automatic reply to an
incoming mail message. Mail is received sent over a protocol which
contains an envelope sender (such as SMTP).
Should the service use the envelope or the header address to choose
the recipient for the automatic reply?
it's a bit of an open-ended question, but in general I recommend
that automatic responses use the envelope return address (or
return-path header address which should be the same). that way,
all responses get sent to the same place, regardless of whether
they are normal replies, nondelivery reports, delivery reports,
or whatever.
Keith