[IMP] [WIP] mail: Set an OpenERP alias as the bounce address.

Attempts to fix issue #3347.

This makes OpenERP 7.0 issue a SMTP MAIL FROM command with an address it "owns".
Is there any good reason to split the `postmaster` and `domain` results? Most of the time the caller will have to join the result before using it as an email, i.e. more code noise. Instead just return a single string in `postmaster@domain.com` form and for the unlikely case where it needs to be split up, the caller will do the `split('@')` call.