Postfix - Email Issues

I am very new to Postfix and for that matter Virtualmin. I have a clean install of Virtulmin gpl on a clean Centos 5.5 OS on a Rackspace Cloud server. I setup the dns records for the domain and created multiple email accounts within Virtualmin with no issues. I can access the email account through Usermin with no issues and I can email to other internal email accounts on the localhost. The issue I have is when I attempt to send emails to external accounts or when I use an external account to email the new email account I setup on Virtualmin I receive an email with the following:

username@domain.com
No Such User Here

I have tried so many things to resolve this issue and I have had no luck, I even attempted to install SendMail but that was not even working for me…

Any ideas?

Try username.domain.com