This may be because of the following reasons:

• The SMTP configuration section was not set up correctly - Check the Gemini's system log.
• You use the scheduler service to send alerts (Administration -> Notifications) but it is not installed.
• You use the scheduler service but did not configure the scheduler config file with the correct URL or a valid Gemini username and password.
• Some users may have the alerts blocked by their spam filter / mail sweeper.
• You are using windows authentication for the SQL connection.