After entering login info into OWA through IE6 browser, user receives the error: HTTP/1.1 503 Service Unavailable. I have Exchange 2K running on W2K servers in a front/back-end configuration. This error message occurs intermittently with no event log reference. Reboot of the front-end Exchange server resolves it for a while and users can access their mailbox through OWA. Then for no obvious reason, it happens again. The front-end server is in a DMZ, no configurations changes have been recently made to the firewall. This problem has been occurring for months and appears to be growing more frequent!