Virtual domain plus catchall user

[Zak McGregor]

Hi all

This _should_ be easy, but for some reason I can't see the answer, can anyone
help please?

Basically, I'm running sendmail + cyrus pop, with virtual domains set up for
email. What I need to happen is user1 at domain.tld gets user1 at domain.tld email,
plus user2 at domain.tld, and the remainder of the emails for domain.tld get sent
to user3 at anotherdomain.tld. 

I think I'm missing something obvious, but nothing I've read seems to discuss
this kind of setup.

Any help at all greatly appreciated, thanks!

Ciao

Zak