If a matching sender's e-mail is not found, then the current user is assumed to be unknown, if unknown exists

The e-mail is immediately rejected

If the sender's email address is not part of email contributor's group, then the email is rejected

If a matching recipient's e-mail is not found, then the current user is assumed to be unknown, if unknown exists

If the recipient's e-mail is not part of email contributor's group, then the email is rejected