MSExch5.5+WinNT4SP6a - POP3 13003 Auth Errors

[techer]

Remote clients trying to log on to my Exchange 5.5 SP3 Server are getting their mailboxes displayed with no new content since Wednesday 21st May. On that day, I attempted to recreate the Exchange service account. After verifying its settings, group membership and user rights, I re-verified its permissions on the Exchange Organisation/Site/Configuration/Server objects within Exchange Admin.

Despite all this, I am receiving the following error in the Application Log;

Event ID - 13003
Source - MSExchange POP3
Description -
Logon attempt from xx.xx.xx.xx to mailbox_alias has failed: HrLookupCredentials() call failed with error A required privilege is not held by the client..

Users cannot see any new mail (remotely) - MAPI connections to their mailboxes show all new mail.

Also, I have set up a mail profile for the Exchange Service Account using Outlook 98 on the exchange server (as required by my Backup agent). When I try to run Outlook98 with this profile, I receive the following DR Watson error.
"An application error has occurred and an application error log is being generated.

OUTLOOK.exe

Exception: access violation (0xc0000005), Address: 0x627eeb1

Nothing corresponding to the above DR Watson error shows up in the Event Log.

Thanks for your help!

Mar