We’ve had a request from a member of our forum to be able to reply-via-email using a different address than the one with which they are registered.
The reason being: they registered with a single-use email address for the forum, which then forwards to their primary email address, which is where they would like to reply from.
They current receive the following message when trying this:
We’re sorry, but your email message to replies+4f7j2e5cdae85fe2333e12d8f9e1fecf@foo.bar (titled Re: XXX) didn’t work.
Your reply was sent from an unknown email address. Try sending from another email address, or contact a staff member.
If you’re using local logins they can add an alternate email to their account (see Discourse Meta - The Official Support Forum for Discourse). It rather obviates the point of creating a single-use email just for the forum if they just want to use that address anyway. Now the forum has two of their email addresses.
Old topic, but I recently had to do this myself for some testing accounts.
Most email clients let you designate alternate send-from addresses for exactly this reason.
Anyone like OP’s user who’s employing forwarding email addresses should look into how to work with them.