We are using ldap for around 90% of our current users.
3 registered user accounts had the issue that when you clicked on them from the user list, you got a http 500 page. You couldn't raise a ticket on their behalf and couldn't do any changes to their account through the web inteface.
In an attempt to remove them manually from the database, I was going through the various user tables. When I deleted their entries out of the user_account table, we were able to access their details in the user list. Once i'd done that with all 3 users, next time the cron job ran, it cleared the mail inbox correctly
