To create an alias for an email address, use the E-Mail Forwarders option in the Email Management Tools of the control panel.
For a forwarder for a single email address, use the Email Account Forwarders option, and Add Forwarder.
Enter the name and select the domain for the email you wish to alias / forward, then enter the email you wish the email to be forwarded to in the Forward to the email address field.
You can also create a domain forwarder where all email for one domain would be forwarded to the equivalent email address on a different domain.
Note that for single email forwarders and for domain forwarders, there is no requirement for the destination email addresses to be hosted on the same server - they can be any email address.