Remove `+` from all email addresses before handling

[Bisht13]

The relayer should remove all + concatenations in an email address such as abc+123@xyz.com should be cleaned to be abc@xyz.com before processing the mail.

[Divide-By-0]

If it's possible to send an email from x+y@gmail.com, I don't think we should do this. Someone should try adding that as a sender address and see if it passes. If not, then we should definitely do this -- but maybe it's better to do on emailwallet.org?