Allow overriding the SMTP envelope from

[arjan]

The SMTP MAIL FROM command can specify a different address than the one on the From: header. This address is called the 'envelope from' and is used by MTAs for reporting bounced emails, amongst others.

This patch allows overriding the envelope from address by putting an email address in the :envelope_from key of the private data of the Bamboo email struct.

reference: https://www.xeams.com/difference-envelope-header.htm