There are two possible methods to forward messages to an external address:
Use this method if it is for a person who might want to receive their email whilst in the office and/or get the mail sent to an external address.
Create a VPOP3 mailbox for the user as normal
Edit the User and go to the 'Routing' tab. Type the external email address into the 'Assistant' box (if you want to keep a local copy of the message) or the 'Forward To' box (if you don't want to keep a local copy)
Use this method if it is for a person who will never want to receive their email whilst in the office. (for this example, we'll assume you want mail sent to fred@yourcompany.com sent to fred@aol.com)
Create a 'Distribution List' called fred
Add a 'Remote Member' to this list called fred@aol.com
Ensure that 'Allow Remote Access to List' is enabled in the list definition
Note that forwarding mail can be problematic with your ISP, or the recipient mail server thinking the message sender has been 'spoofed' (faked). Instead, a much better way to allow people outside the office to collect mail is to allow remote access to VPOP3 mailboxes
Also, see: Forwarded emails not arriving