It appears that the incoming mail processor correctly handles mail when the address is in the To: field, but rejects mail when it is in the Cc: field.
Upon sending a message with headers like this:
delivered-to: phabricator+D12345email@example.com received: Array x-received: Array from: Jane Doe <firstname.lastname@example.org> date: Tue, 18 Nov 2014 11:52:36 -0800 message-id: <email@example.com> subject: Re: [Request, 666 lines] D12345: Frobnicate the bazzifier to: Joe Smith <firstname.lastname@example.org> cc: phabricator+D12345email@example.com
Users report getting this bounce:
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.)