If you did not reach the 3 domain limit, you can add a new one, make it the default domain, and then delete the old one. After the old one is deleted, readd it, reset it as default, and delete the temporarily added one.
https://gjr.x10.mx/#mail
Port 587
STARTTLS encryption
User name would be the full noreply@mydomain.com
Password would be the password you configured for that account when you created it.