From address always added to Reply-To even if Reply-To is set

I am also having this problem. Will try that suggested fix.

Yup, this is an issue!

I think we should keep the call to $mailer->setFrom() as-is though; if a Reply-To header is not present, the default should be whatever the "From" address is. Therefore, my proposed solution is to just clear out any set Reply-To headers before we explicitly add them. Patch doing just that is attached.

We replaced what we did with the patch in #4 and the issue has not returned. I can confirm the patch works.

We've had the patch in #4 deployed in our production environment for the last 30 days, sending thousands of emails each week successfully with Reply-To header sets correctly.