I'm not sure which version you are on but if you check the Email queue, what is the status of the emails (Failed, waiting, etc)? If they are failing, have you checked the error info for any idea on what is going wrong? How about any errors in the Event log?
In general, I would recommend reviewing this documentation page for some things you can check: Troubleshooting marketing emails along with verifying your SMTP settings are correct (Configuring SMTP Servers). You can also try sending a test email in the System application -> Email tab to see if you can get anything sent from there.