Reply-To Header Notes

Written by Daniel Eriksen as a presentation for
Bruce-Grey Linux Users Group
February 26th, 2005

    While I have understood the low level specifics in how email is sent and received for quite awhile, there have been a few things that I never quite grasped (and still don't).  The following is simply what I have picked up recently from observing the behaviours of the mailing list I help run and the many examples of forged headers I see in my spam.
    Much of this is seems fairly basic, but anyone who has had to work with the intricacies of email knows that email handling is incredibly complex (judging by the multiple RFCs and their length).  Also note that this is my interpretation of things.  There are others out there that believe that things are meant to operate differently.

About the Reply-To header

    The From header is supposed to contain the address from which you are sending the email.  If you desire to have the receiver reply to a different address then the one that you are sending from then you can specify this different address with the Reply-To header.  Many people are setting the Reply-To header to always give the same address as the one that they send from (this can be specified in the mail clients account settings).  This is not what it was intended for and can result in odd behaviour.

    For example, if you send an email to a mailing list and someone hits reply to that email they will end up replying to your address instead of the mailing list address.  This can be handy if you actually do want people on a mailing list to reply to you directly, but having it always set to this can be annoying for those that are trying to participate in a mailing list discussion.

About altering Reply-To Sender (mailing list)

    So now you ask "why not have the mailing list set a Reply-To header automatically?"  Well, first it would kill the ability to legitimately set a Reply-To header (unless the mailing list software were to only add a Reply-To when it was missing).  Second, it will probably confuse other email software.

    Take auto-responders for example.  If someone accidentally has an auto-responder set and forgets to set it to ignore mailing list traffic and the mailing list is adding it's own Reply-To header, then the auto reply will be sent to the list, which will then send it back to the person auto-responding, which then sends an auto-reply to the list, which then...
    But if the list does not munge up the headers, an auto-reply response will instead be sent directly to the person who sent the original email.  The best rule of thumb for mailing lists is to mess around with headers as little as possible.

    Another example would be the situation where someone wishes to send an email on behalf of someone else, so they specify their address in the Reply-To.  If the mailing list over-writes this header, then the intended sender's email address is now lost.

    Many mail programs are becoming aware of mailing lists and (at least with Sylpheed) have an option to automatically reply to a mailing list by default (except when a Reply-To header says otherwise).  My reply-to button allows me to choose whether I want to reply to Sender, All or List.  How does the mail program know that the email in question is from a mailing list?  Well, there are a bunch of headers that are added for mailing list mail like List-Id and List-Help.  If your mail client does not understand lists, then you can always use Reply All or Reply Group.

To Conclude

    Consider having your email client only set the Reply-To header when you actually wish to change the return address, or when you want people on a list to reply to you personally instead of the list.

Reading Email Headers
How to Interpret Email Headers
Why Reply-To munging is bad