If you are having problems connecting or sending emails through your SMTP server, the SMTP class can provide more information about the processing/errors taking place.
Use the debug functionality of the class to see what's going on in your connections. To do that, set the debug level in your script. For example:

$mail->SMTPDebug = 1;
$mail->isSMTP();  // telling the class to use SMTP
$mail->SMTPAuth   = true;                // enable SMTP authentication
$mail->Port       = 26;                  // set the SMTP port
$mail->Host       = "mail.yourhost.com"; // SMTP server
$mail->Username   = "name@yourhost.com"; // SMTP account username
$mail->Password   = "your password";     // SMTP account password

Notes on this:
$mail->SMTPDebug = 0; ... will disable debugging (you can also leave this out completely, 0 is the default)
$mail->SMTPDebug = 1; ... will echo errors and server responses
$mail->SMTPDebug = 2; ... will echo errors, server responses and client messages

And finally, don't forget to disable debugging before going into production.