Postfix help

[lpeet at eccod.com]

Ok, now that some of you convinced me to go to Postfix for my mail server,
I have aquestion for you:

Postfix installed flawlessly, and is working great.  Only trouble is, when
I add a new user tomy system, and try to send mail to them, I get the error "Mailbox not
found: 550 error - Userunknown".  Sending mail works fine for users who were on the system
pre-postfix install.
Any ideas?