I am surprised by all the apologetic replies about mailing lists,
when "Reply-To:" is in fact standardized for more than 30 years,
and it has nothing to do with mailing lists.
Wietse
Citing from RFC 5322 section 3.6.2 (published 2008):
When the "Reply-To:" field is present, it indicates the address(es)
to which the author of the message suggests that replies be sent.
In the absence of the "Reply-To:" field, replies SHOULD by default
be sent to the mailbox(es) specified in the "From:" field unless
otherwise specified by the person composing the reply.
Citing from rfc2822 section 3.6.2 (published 2001):
The originator fields also provide the information required when
replying to a message. When the "Reply-To:" field is present, it
indicates the mailbox(es) to which the author of the message suggests
that replies be sent. In the absence of the "Reply-To:" field,
replies SHOULD by default be sent to the mailbox(es) specified in the
"From:" field unless otherwise specified by the person composing the
reply.
Citing from RFC 822 section 4.4.4 (published 1982):
o If the "Reply-To" field exists, then the reply should
go to the addresses indicated in that field and not to
the address(es) indicated in the "From" field.
