Sub-Select in CASE

Hello all,

I am trying to run a query that checks to see if a mail box exist in a
db. If the mail box does not exist i need it to return another address.

Currently I am working with :

SELECT CASE WHEN alias.mbox='brian' THEN alias.email_add ELSE (SELECT
alias.email_add WHERE alias.mbox='*' AND alias.domain='domain.com) END
WHERE alias.domain='domain.com' and alias.mbox='brian';

I am not even sure if Case is the best choice.

More background info on it, this is for and db that EXIM enter acts with
and i want to be able to setup a default alias.