Should the service use the envelope or the header address to choose
the recipient for the automatic reply?
The important distinction is between a reply by the transport service,
about transmission, versus a reply by an email-based application, about an
activity of the application.
Certainly a reply by the mail transport service should always go to the
return-path / MAIL FROM address - that much, at least, is not controversial.
However it's not at all clear that a reply that is not from the transport
service should go to the reply-to address(es) even if the field is present.
For example, automatically generated message disposition notifications aren't
generated by the mail transport system - they're generated by the recipient's
user agent - but they are still supposed to go to the return-path address.
from RFC 2298:
MDNs SHOULD NOT be sent automatically if the address in the
Disposition-Notification-To header differs from the address in the
Return-Path header (see RFC 822 [2]).
As to whether Reply-to should be used for other kinds of automatic replies,
the answer can be subtle. If the sender is a human, and he/she is expecting
an automatically-generated reply, then it might be reasonable to assume
that if the human set a reply-to field then he/she intended the reply to
go to that address rather than the From address. On the other hand, if the
sender is not expecting an automatically generated reply, but is expecting
a reply from a human (as in a "out of the office" response) then sending
to the return-path address usually makes more sense. Consider a message
intended for a mailing list where the sender specified the list address in
the reply-to field (which is a perfectly reasonable thing to do) - if a robot
answers the mail on behalf of a list recipient and sends the reply to the
entire list, this will be seen as disruptive.
And if both the sender and recipients are computer programs - i.e. if
this is the use of email within an application - it's probably best for
the sender to never use reply-to (and for the recipient to ignore it).
The bottom line is that the specifications leave a lot of wiggle room
regarding use of Reply-To because we really haven't been able to find
a clear set of rules that works for all cases.
Keith