Please check whether or not you satisfy the requirements for SMTP sending. Most of the time this is cause by your hosting not allowing outgoing traffic to port 25 (PHP is not able to communicate with your SMTP server).
If you are interspire.smtp.com user, the following port can be used to connect to interspire.smtp.com server:
- Port 25
- Port 2525
- Port 25025
- Port 2082