OUTLOOK SENDING TO PREVIOUS ADDRESS IN CACHE

I had a issue one of my users needed his email account forwarded to a email address outside the organization.  He kept saying it wasnt working (i later found it was his junk mail filter).   Because i thought it wasnt working for him i setup a test account on the server called forward test.  I setup this email account to get forwarded to a outside address.  I setup another account already on the server to forward to the forward test account so the mail would actually forward outside the organization.  Long story short i got that issue resloved.  So i then removed the forward test account.  The problem is, every time users on that netwrok use outlook to send a message to me they get a undeliverable error (becuase its still trying to send to the forward test account even though its been deleted off the server.  PLEASE HELP!