Our Microsoft Exchange server is experiencing failure audits generated
by users logging on through Outlook Web Access. The errors occur
repeatedly while the person is logged on, so the security event log
fills two or three times a day.

Source: Security
Category: Account Logon
Type: Failure Audit
Event ID: 680
User: NT Authority\ System
Computer: Adams

NT Status Code: 0x0000064 (account does not exist)

This error occurs in conjunction with Success Audits 576 and 540. This
trio of events occurs repeatedly while the user is logged on. It
occurs when users are logging on either inside or outside the domain.
The errors do not occur when the user is logged on to the domain and
uses Outlook to check e-mail. Please reply to e-mail. Thanks.