Could you please state what kind of setup you are running (Debian,Ubuntu...). Did you follow any of the "Perfect setup" tutorials? What SMTP server do you use (Exim,Postfix,Sendmail) and where did you get the errormessage, in your e-mail client or in a server log?
When you say that you created an alias for the site you are mixing two concepts. An alias is an e-mail address alias and when you say an alias for the site I get the impression that you created a co-domain.
If you are running postfix, check your /etc/postfix/virtusertable and check if you got both addresses in there.