MX Toolbox is a MX record and DNS look up tool. You can find the current MX records and mail servers.
In the MX Lookup text box enter the domain without the http://www. example mydomain.com.
If you are using Google App as your mail box the MX record will look as below:
Priority Mail Server
1 ASPMX.L.GOOGLE.COM.
5 ALT1.ASPMX.L.GOOGLE.COM.
5 ALT2.ASPMX.L.GOOGLE.COM.
10 ALT3.ASPMX.L.GOOGLE.COM.
10 ALT4.ASPMX.L.GOOGLE.COM.