SMTP is used as a last resort alternative when neither the plugin’s send_mail(), nor the standard PHP mail() function works to send out emails from within the plugin. There is no guarantee this option will work.
To use this option you need to go to your classifieds admin options page under SMTP Settings and provide your SMTP details.
The system will attempt to use the first two methods to send out emails and if both fail and you have provided your SMTP details it will try to use SMTP to send the emails.
If SMTP also fails it is recommended you contact your hosting service provider for assistance in solving the problems you are having getting the emails to go out. If you know for absolute certain that other plugins you use are successfully processing emails you can contact using the contact form and every effort will be made to figure out a workable solution on your behalf; but if this is strictly a server issue, you will need the help of your hosting service provider in solving the problem.