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