Users sometimes seem to mistype their mail address.
We get notified about that by errbit when the domain part is not resolvable, e.g.:
Expected response code "250/251/252" but got code "556"
Recipient address rejected: Domain googlemial.com does not accept mail (nullMX)".
- Investigate on how to suggest the user the proper spelling of common mail provider names.
- add "confirm email address" field that compares both fields and hints the user about input errors