Personally, I would never use code like this example below.
<?php
# Code for illustrative purposes only. This should
# not be used with malicious intent.
$counter = 1;
//$targetEmail = "unpopular.person@someplace.cock";
//$mailFrom = "Someone you've annoyed";
//$mailTo = "Target <$targetEmail>";
//$mailSubject = "We do not like you very much";
//$message = "Insert email message here...";
while($counter <= 100)
{
//mail($mailTo, $mailSubject, $message, $mailFrom);
echo $counter . "<br />";
$counter++;
}
?>
Upload and run it and you'll see it echos the value of $counter to the screen 100 times (really quickly). Now imagine what would happen if you removed the echo and un-commented the rest and increased the condition in the while block to 1000 or more
Again, I'd stress that I'd never go to an internet cafe, upload this to some free webspace and run it from there. I'm pretty sure that's what the bad folk would do.