As per [RFC 5321](https://tools.ietf.org/html/rfc5321) section 3.6.3, a `MAIL FROM:<>` command is valid and depicts a bounce message. I'm actually not sure if we should allow this. Maybe make the valid sender addresses a regexp, too?
As per RFC 5321 section 3.6.3, a
MAIL FROM:<>command is valid and depicts a bounce message.I'm actually not sure if we should allow this. Maybe make the valid sender addresses a regexp, too?