You are more than welcome to use an external mail service while hosting your sites and applications with us. We fully support the use of GoogleApps however you are welcome to use any mail provider you wish.
Directions on setting up GoogleApps via cPanel:
1. Log in to your cPanel account.
2. Under the Mail header click MX Entry.
3. Choose the domain for which you wish to use external email.
4. Three new sections will show up and under the Email Routing section you will want to select Remote Mail Exchanger.
5. Under the Add New Record section you will want to add your external mail server records.
For Google Apps you can see a list of servers at http://www.google.com/support/a/bin/answer.py?answer=174125
6. Once you have added your external MX Entries, you will want to make sure to remove any other entries.
Congratulations! Your MX records are now configured to point to your external mail provider. Keep in mind that changes to MX records may take up to 48 hours to take effect.