The Mirroring - Unsent Log alarm is raised when the size of an unsent mirroring log exceeds a threshold.
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This alarm may indicate a problem with the link between the principal and mirror databases.

Mirroring alarms are disabled by default. It is important to configure alarm thresholds that are appropriate to your environment before enabling these alarms.

Use a Spotlight Client to Configure | Alarms. You can configure this alarm to ignore certain values. You can configure different alarm severities for specific principal databases. The key for this alarm is the name of the principal database.