How can a raised alert fail to notify an operator via e-mail? SQL Server 2008: A message with @msgnum = 75000 and @severity = 16 was added to the SQL instance. An alert was added that is raised when error number 75000 is raised. The alert is enabled and an operator, who is also enabled, is set to be notified via email. The alert history shows that the alert is triggered, but the operator is not notified. The operator history shows "Most recent notification attempts: (Never e-mailed)".

The operator has a correct e-mail address. DBMail is setup correctly since I can sp_send_dbmail from and to that e-mail address.

Where do I begin to troubleshoot?
You might check the on duty schedule for that particular operator.  Did this alert happen while the operator was not on duty?

Is SQL Agent set to use Database Mail and a specified Mail Profile for the Alert System?  (Right Click SQL Server Agent > Properties > Alert System)

These are two things that I can immediately think of that could cause this behavior.

Hope this can help.