When receiving bounced message after sending out an e-mail with such error on e-mail header:
SMTP error from remote mail server after end of data:
host mail.domain.com [xxx.xxx.xxx.xxx]: 554 5.7.1 This message has
been blocked because the return email domain is invalid.

Double check or recreate a new mail settings, make sure they are correct as follows:

1. Incoming Mail Server: mail.yourdomain.com (port 110)
2. Outgoing Mail Server: mail.yourdomain.com (port 25)
SSL NOT required for incoming & outgoing

3. Username: you@yourdomain.com
4. Password: *your password*
Login using secure password authentication NOT required
Make sure e-mail address is a POP3 settings

5. Reply address: you@yourdomain.com (This is the return-path)

Please create a new ticket for further technical assistance