You would like to have email on your domain alias in addition to your main site.
While this is an option, we recommend that you use a 3rd party like Google Apps for the portal (and all its email). With Google Apps you can use your own domain. It will not be a Gmail account. This article explains How to Configure Google Applications.
Email Setup on a Portal
Some background information: the primary domain example.com is registered with the email server. All domain aliases created under this default portal will show up as example.com to the mail server provided that the Mail box is checked when creating the domain alias. If you have a portal named site.com and you want an email like firstname.lastname@example.org, you would create the account email@example.com, and it will be able to send mail from and accept mail for firstname.lastname@example.org. You would access the account through email@example.com normally, and mail sent to either address would arrive to the inbox. Understand that when set up this way firstname.lastname@example.org and email@example.com are just different names for the same account. This is not a viable alternative if you require that the addresses for the different domains be entirely separate accounts.
If you need assistance on setting up email accounts through your Control Panel you may follow the steps included in our article titled How to Set Up Email Accounts in our Environment.
We are a CMS specialty hosting company; we are not email specialists. We offer a basic email service. If your environment requires sending a significant number of emails, or the use of mailing lists, newsletters, or other bulk email you will likely be disappointed with this email service. For more sophisticated email requirements we strongly recommend Google Apps, or any other email specialist you may choose.