Change the send-as functionality to use a string in the Subject.

Change the send-as functionality to use a string in the Subject.

Using the RCPT TO for BCCs does not work if mailpopbox is not the
original MTA to accept the message. The originating MTA creates two
envelopes with 1 RCPT TO each, rather than 1 envelope with 2 RCPT TO.

Editing the subject breaks Gmail threading on the send-side, but it
works. An alternative was to search the body of the message for a
string, but that gets more complex with complex message Content-Types.