Your MX records should be its assigned to you when you set up your domain for the filtering. In general, your MX records should follow the following format:
yourdomain.com. IN MX 10 yourdomain.com.pri-mx.smtproutes.com. yourdomain.com. IN MX 90 yourdomain.com.bak-mx.smtproutes.com.
The specific MX records will be emailed to you in the Welcome email when the account is setup.