Net_SMTP isn't installed on your server go to http://pear.php.net/package/Net_SMTP/download download the file.

Unzip the file, then upload it to a directory on your server.

SSH into your server as root, type cd /then/the/directory/location/you/uploaded/to/ hit Enter

Then type pear install Net_SMTP-1.6.2 and hit Enter

You should see it spit out info it was installed. Go back and run your php file again and the problem should be fixed.