repro case:

• phabricator instance with mailgun reply handler configured
• create a Phriction document and add a subscriber

expect:

• subscriber gets an email from noreply@ and with no reply-to: field

actual:

• subscriber gets an email notification with Reply-to: WIKIasdfasdf@instance
• user takes the presence of a reply to (obvious when you go to reply to the email) to mean it can be replied to
• replying to such email causes the following email to be delivered to you:

Subject: Error Processing Mail (No Receivers)

Your email to Phabricator was not processed, because an error occurred while
trying to handle it:

Phabricator can not process this mail because no application knows how to
handle it. Check that the address you sent it to is correct.

(No concrete, enabled subclass of PhabricatorMailReceiver can accept this
mail.)