Postfix Relay Access Problem

Using PostFix for SMTP under RedHat. Using POP-before-SMTP authentication. Sometimes a user will start getting Relay Access Denied messages every time they try to send to another domain. Then, the problem will go away by itself a half-hour later: the user hasn't quit his email client, and the server hasn't been kicked. I've seen this with Mozilla and MsEntourage clients. Moz is checking email using IMAP, and Entourage is using POP.

Anyone know what's happening?

I posted a UseNet http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=e2491a75.0401021409.2e490b57%40posting.google.com&rnum=2&prev=/groups%3Fq%3D%2522bill%2Bseitz%2522%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26sa%3DG%26scoring%3Dd |thread