Post History

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](https://www.rfc-editor.org/rfc/rfc5322.html#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](https://marc.merlins.org/netrants/reply-to-harmful.html) 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](https://www.rfc-editor.org/rfc/rfc822.html#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](https://www.rfc-editor.org/rfc/rfc2369#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](https://www.rfc-editor.org/rfc/rfc7489.html) 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.