guest user should not be disabled in the msdb database in SQL Server
SQL Server Books Online recommends that you disable the guest user in every database as a best practice for securing the database server. This recommendation does not apply to master, msdb, and tempb system databases.
In order for some Microsoft SQL Server features to work, the guest user must be enabled in the msdb database.
More info: http://support.microsoft.com/kb/2539091
