Now many Magento hostings don’t support free email service, so we have to use other free email service for Magento, which is necessary to send transactional emails, such as Sendgrid, Mailgun, Mandrill etc.
Magento 1.x setup mailgun free email service
There is different details for domain setup matching email service suppliers, we can follow their instructions.
Here, we take mailgun for example to setup on Magento:
System-Configuration-Advanced-System-Mail Sending Setting
Note: Normally port 25 are not supported, so we can use 587 or 2525.