I'm not sure how you coded your php mailer but it looks like a standard form. I'm assuming that the code is something like the following:
$to = 'your friend's email addr';
$subject = 'blah blah';
$message = 'content ,etc...';
$headers = 'From: firstname.lastname@example.org
' . "\r\n" .
' . "\r\n" .
If I'm wrong stop reading now.
Anyway the problem occurs with the using $_POST['subject']; for example.
This in and of itself is not a bad thing but the "\r\n" specifies a new line and lets the smtp server know another header is being sent. So what we, and by we I mean the spammer, can do is to enter something like
\r\nTo: email@example.com; another address, etc...
in the subject field. This then gets interpreted as a new header and your form is now being used to send spam.
Venom77 is right and you need to make sure that your code is sanitizing the input. Use a regular expression and validate the input and strip out unwanted characters. Replace all characters that are not alphanumeric or spaces.
I would also make sure that you check the "tell a friend" form.
BTW your Keyword search field is susceptible to XSS. http://businessconnectory.ca/display_results.php?key=Search&search=%3Cscript%3Ealert(%22test%22)%3C/script%3E&found=keyword