Emails not sending in Magento

There will be many scenarios for the email sending issue in Magento.

1. Check if email sending is completely disabled in Magento: System => Configuration =>System tab ... Under Mail Sending Settings sub section "Disable Email Communications" should be set to NO
2. Does Magento sending other native emails (new registration, order confirmation etc)?
3. Check if the email sending is enabled for shipments in Magento admin: System => Configuration => Sales Emails => Shipment Comments should be enabled (By default it is enabled)
4. Is Magento using any extension to send the emails? If this is the case click here
5. If Magento is not using any extension to send the emails then we need to check the server whether it is supporting the email sending or not. 
6. If the issue is in 1.3.1 version then the solution will be found here