The application event log on our Exchange 2003 server is full of error Event ID: 1022 (attached).
They start at 7p every night and run until about 12a -- repeating the error for the same dozen or so users over and over about every 10-20 minutes.
Looking at the accounts, they do appaer to be newer accounts or special accounts that aren't used often. Maybe it's account that haven't been logged into yet?
We use Symantec BackupExec on this server.
Microsoft KB suggests making sure the accounts are not locked or disabled - no fix there.
Another post on here said a SP install fixed their problem, but I'm already on SP2 for both the Server and Exchange.
Logon Failure on database "First Storage Group\Mailbox Store (SUNNA)" - Windows 2000 account NT AUTHORITY\SYSTEM; mailbox /o=Dana College/ou=First Administrative Group/cn=Recipients/cn=doe.john .