The Monitoring and Alerting Infrastructure provides the enhancement spot ALERT_REACTION where you can specify custom alert reactions.
This enhancement spot and the corresponding Business Add-In (BAdI) BADI_ALERT_REACTION are described in the document MAI Alert Consumer BAdI Interface.zip.
This functionality can be used to trigger additional events in case of an alert like for example:
There is also a BAdI interface which allows you to change the content of an email notification before it is sent. This interface is described in the SAP Note 1747633.