Get tests passing on python 2.6.8 / 2.7.4.

Apparently BBB across point releases is not *that* important.
Normalize imports.
Accept non-duplicates w/ 'Message-ID' same, but different 'X-Original-To'.

On systems which add 'X-Original-To' (postfix adds it by default), its
presence indicates delivery to a separate user; we should allow it to be
queued for attempted processing.