@domain1 valid_user@domain1

That's a really bad idea. Just add hostmaster@ webmaster@ abuse@ postmaster@

Catch all mailboxes are SPAM magnets as many spammers cycle through common and some really really odd usernames to find mailboxes to spam. You'll have many thousands of SPAMs in your mailbox if you permit that and have no SPAM checking place.

I've seen a catch all mailbox get 10,000 SPAMs a day.

Thanks

Jason