PHP Mail Transport to allow all recipients in BCC

It is sometimes usefull to send emails with all recipients as BCC.

The ESMTP transport does allow this usage, while the Mail transport doesn't.

In commit https://github.com/swiftmailer/swiftmailer/commit/476586b810a98d06be477fd01c4309c74993e1b4, the fix was to get rid of a fatal error in two situations: no recipient in 'TO' or missing 'Subject'. That fix then consisted in allowing a missing Subject, but sending an exception in case there is no recipient in TO.
My proposal is to allow both usage in the end.

Regards,
Romain
This exception should still be thrown if there is no recipients