add generate_ndr to PMG::SMTP
In order to selectively accept and reject mails when multiple recipients
are present for a mail, without silently dropping mails to certain recipients
the sender needs to be informed about those recipients which did not receive
the mail (via a non-delivery report - NDR).
The format of delivery status notifications (of which an ndr is a subset) is
specified in RFC 6522 [0]. The format for the 'message/delivery-status'
MIME-Type is specified in RFC 6533 [1]
The message text was adapted from Postfix' default bounce-messages, however
we do not attach the original mail to the report. This is acceptable by [0]
and makes sense (not sending mail, which is likely spam or a virus, back to the
(potentially faked) sender).
[0] https://tools.ietf.org/html/rfc6522
[1] https://tools.ietf.org/html/rfc6533
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>