To forward all email when it arrives (excluding any email explicitly caught by discard rules, or categorised as spam): 1. Open the Settings → Rules screen and click New Rule in the toolbar at the top. 2. In the Conditions section, click The sender's email and change this to All messages. 3. In the Action section, select Send a copy and enter the email address you want to forward to. To target multiple addresses (for example, an external primary and a Gmail for backup), separate the addresses with a comma. 4. Click Save in the toolbar at the top.
Rules also let you only forward messages that match certain criteria.
If you just want to forward all mail sent to a particular alias, you can do this by directly setting the target of the alias to the address you wish to forward to.
The main differences between this approach and using a rule are:
If these limitations on virus, backscatter, and spam filtering are OK, then you can use the Settings → Aliases screen to target aliases at the external address(es).