If you’re using DreamHost for email, go to the Mail: Custom MX section of the DreamHost Panel, click the “edit” button for the wrong domain, and set that domain to “mail mirror” the main domain. This will make every address that’s valid on the main domain also valid on the domain that’s mail-mirroring it, and make it deliver to the same mailbox.
If you’re using an external mail provider, you’ll probably have to set up a bunch of forwards… sorry. We can’t forward every address on the domain, since that would include a lot of junk mail to nonexistent addresses, and we don’t know which addresses need to be forwarded without you setting them up. You may want to check with that mail provider to see if they’ll let you add both domains to your account with them.