Allow to set FQDN to be used for message id

When using django-mailer I recognized that created mails are not having a real FQDN (e.g. from Django's site setting) but are created with the local hostname. This is not a real issue, but not exactly completing RFC 2392. 

It would be really cool to have a setting or more reliable logic for autoamtic detemination of the FQDN used at the message id field of a created email. 
