User unknown in local recipient table

I have two users who can not check their mail (via web mail nor mail.app), nor can mail be sent to them. They can log in to their network accounts though. Running 10.5.5 on xServe. Messages in the mail server log reads: May 23 20:42:26 tiger postfix/smtpd[2258]: NOQUEUE: reject: RCPT from 97-119-71-38.omah.qwest.net[97.119.71.38]: 550 5.1.1 : Recipient address rejected: User unknown in local recipient table; from= to= proto=ESMTP helo= May 23 20:58:40 tiger postfix/smtpd[2435]: NOQUEUE: reject: RCPT from unknown[192.168.1.142]: 550 5.1.1 : Recipient address rejected: User unknown in local recipient table; from= to= proto=ESMTP helo=