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+D12345+public+0deadbeef991@mail.internal
received: Array
x-received: Array
from: Jane Doe <jdoe@mail.internal>
date: Tue, 18 Nov 2014 11:52:36 -0800
message-id: <77c1521bee8562922bf5159c9afd3c54@mail.internal>
subject: Re: [Request, 666 lines] D12345: Frobnicate the bazzifier
to: Joe Smith <jsmith@mail.internal>
cc: phabricator+D12345+public+0deadbeef991@mail.internal
```
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.)
```