In Central or MSP server, the change alerts are sent using the alarm mailer thread which will be launched when the SEND_ALERTS_FROM_SERVER setting key is set to 1. This can be set using Advanced System Settings->Global Settings. If this is already set to 1, then please check whether SMTP configuration is correct.
If this key is set to 1 then alerts are sent from central server, 0 means alerts are sent from probe. In other setups, this setting key will not be considered