PHP Mail Transport to allow sending email 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 handle to get rid of a fatal error in two situations: missing 'TO' or missing 'Subject'. That fix was to allow a missing Subject, but send an exception in case there is no recipient in TO.
My proposal is to allow both usage in the end.

Regards,
Romain