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Can an email's Reply-To have a different domain than its From? Question
Are there any negative effects[1] when sending an email wherein the From header is an email address on my server's domain but the Reply-To header is an email address on a different domain, specifically one whose DNS I do not control?
I've heard conflicting accounts. Some people say "yes;" some say "no;" and some say "only AOL" or "not if they share a public suffix."
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Deliverability, email client warnings, etc. ↩︎
1 answer
Any such negative effects would seem to be against at the very least the letter, and really also the spirit, of RFC 5322 section 3.6.2 on originator fields.
In the words of RFC 5322:
The "From:" field specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message.
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.
Note that there is specifically no indication that they need to list the same set of addresses. (Both the From: and Reply-To: fields can list sets of addresses, although single addresses are probably by far most commonly seen in practice.)
Quite a few email discussion lists do reply-to rewriting, even though there are examples going back to the late 1990s on why this practice is harmful. It's worth noting that the language in RFC 5322 is subtly different from that of the original Internet email format standard (RFC 822), which in section 4.4.3 says, in part:
This field provides a general mechanism for indicating any mailbox(es) to which responses are to be sent.
A somewhat different use may be of some help to "text message teleconferencing" groups equipped with automatic distribution services: include the address of that service in the "Reply-To" field of all messages submitted to the teleconference; then participants can "reply" to conference submissions to guarantee the correct distribution of any submission of their own.
Note the difference between "a general mechanism for indicating any mailbox(es) to which responses are to be sent" and "the address(es) to which the author of the message suggests that replies be sent". (RFC 2369 section 3.4 largely accomodates the "text message teleconferencing" use case mentioned in RFC 822.)
So yes; it is, and has always been, valid for the From: and Reply-To: headers to indicate different addresses, just as it is valid for the SMTP envelope sender and the message From: header to indicate different addresses.
What should probably be watched out for is DMARC policies. DMARC isn't supposed to be used for the Reply-To address, but it wouldn't surprise me if some overambitious spam filtering solutions use the combination as a spam scoring signal. This risk can be mitigated by ensuring that the originating MTA is authorized to deliver email using the domain from both the From: and the Reply-To: headers.
It's also worth a mention that if DKIM is applied, which you probably should do these days if you care about deliverability, any changes to the headers discussed here should obviously be applied before the message is signed. If the message is initially generated using different addresses for From: and Reply-To:, this should not be a problem.
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