Backup backend

Hi!

This PR introduces a feature that allows you to send an email even if the DB is unavailable.

It solves problems when you want to send mail (eg. through cron) but the DB connection is lost. It should also protect against lower-level errors related to the DB, that might go unnoticed, because the email can't make it to the DB, truly a classic situation.

What do you think? Merge ok?