Problem solving

If you cannot send or receive mails in gmail you will need to set the MX record.

So you go to your dns service and you set that the MX record for X.com is mail.X.com a

In most cases all you have to do is set your MX record in the new domain for example if your normal domain is foo.com and your new domain and your normal mail server is mail.foo.com. You would specify the MX record of bar.com to be mail.foo.com