I ran into the same problem in a SQL Server 2012 instance (SP2 applied). I haven't found the root cause yet, but we will be looking at the AD caching...

I just had the same problem. I think there is a bug-report concerning this issue:

http://connect.microsoft.com/SQLServer/feedback/details/675830/spurious-oldest-unsent-transaction-alerts-for-database-mirroring

Are you aware of the security risks? You could be exposing the system-administrator credentials to an end-user. Besides that, this solution works only when your instance is running in mixed-mode...