The following Threshold Alerts, confirmed to be written to the SQL Server Log, need to be defined for the <all databases>** Database Name rather than a single named database:
• Unsent Log Threshold (Error Message 32042)
• Oldest Unsent Transaction Threshold (Error Message 32040)
• Unrestored Log Threshold (Error Message 32043)
• Mirror Commit Overhead Threshold (Error Message 32044)
This is in contrast to the documentation provided by Microsoft:
• Technet: "Alerting on Database Mirroring Events" (http://technet.microsoft.com/en-us/library/cc966392.aspx)
With the thresholds already defined, when one of the thresholds (e.g. Unsent Log: http://msdn.microsoft.com/en-us/library/aa337325.aspx) is exceeded for an extended period of time, the event is written to the log, however no email is sent to the Operator. I think that this is because this event (32042) is being logged as a server-level event and not a database-level event.
** When defined with <all databases>, an email is sent without specifying the database that exceeded its threshold.
See also: http://www.sqlservercentral.com/Forums/Topic1029139-2854-1.aspx