Write down the email address you want messages to be forwarded to - this can be an email address associated with a mailbox at DreamHost. And you know it is associated with a mailbox because the email address appears in the "Mailbox" category when you look at the list of addresses for a domain.
To add one address at a time:
Click on "Create New Email Address"
Scroll down to "Forward Only Email" and fill out the form there to do this one at a time.
To add more than address at a time:
Click on a "Bulk-Edit Forwarding-Only Address" and you will see instructions on what to do. It's not hard. In the text area you type the new address, then add whitespace, then type the address message will be sent to, and press enter. So for four addresses you will have four lines and the second address on each line will be email@example.com
Then never enter "catch-all" as a a fully hosted or forward-only address.