D. J. Bernstein
Internet mail
Internet mail message header format
Response targets: Reply-To, Mail-Reply-To, Mail-Followup-To
The value of a Reply-To, Mail-Reply-To, or Mail-Followup-To field is an
address list.
The use of these fields is explained below.
Note that Mail-Reply-To and Mail-Followup-To were not specified in 822.
Responses
The recipient of a message often sends a message in response.
There are two very common target lists for the response:
- Everyone who saw the original message.
In USENET a response of this type is called a followup.
Other names in MUAs include ``reply to all,''
``reply to sender and recipients,'' ``wide reply,'' and ``group reply.''
- The author of the original message.
In USENET a response of this type is called a reply.
Other names in MUAs include ``reply to author,''
``reply to sender,'' and ``personal reply.''
Replies and followups are the same if there was only one recipient.
The idea of Mail-Reply-To is to specify the target list for replies;
the idea of Mail-Followup-To is to specify the target list for followups.
These are analogous to the Reply-To and Followup-To fields in USENET.
Both replies and followups have been automated,
or at least partially automated, in practically all MUAs.
However, 822 did not recognize replies and followups as separate functions.
It recommended that Reply-To override From as a response target.
Replies
In most cases, replies should be sent to the
From address in the original message.
However, in several circumstances this is not the desired target list:
- ``My From address is usable only for outgoing mail,
not for incoming mail. Use another address.''
- ``This message is from Bob too, but my MUA doesn't
let me put multiple addresses into the From line.
Please send Bob a copy of your reply.''
A survey in 1997 showed that
most MUAs sent replies to Reply-To, defaulting to From.
Exception: BSD mail sent replies to From, ignoring Reply-To.
Unfortunately,
some mailing lists change the Reply-To field in every message.
I recommend sending replies to Mail-Reply-To,
defaulting to Reply-To, defaulting to From.
Followups
In most cases, followups should be sent to the
From,
To, and
Cc addresses
in the original message.
However, in several circumstances this is not the desired target list:
- ``This message is going to the sos-announce mailing list.
Please send followups to the sos-discuss mailing list.''
- ``I'm a subscriber to this mailing list.
Please don't send me an extra copy of followups.''
A survey in 1997 showed that
most MUAs sent followups to the reply address plus To plus Cc.
Exception: BSD mail sent followups to Reply-To alone,
if Reply-To was supplied.
I recommend sending followups to Mail-Followup-To,
defaulting to the reply address plus To plus Cc.
See my
Mail-Followup-To page
for further discussion.