On our shared hosting servers we have had to disable the mail() function for spam prevention purposes. This resulted in many of our IPs being blacklisted and our team having to request IPs to be removed from the blacklist, which resulted in inconvenience to customers that are sending genuine e-mails. As a result customers will have to use a PHP e-mail library in order to send mail, such as SwiftMaier or the Mail class in the PEAR library that allows you to send mail directly over the SMTP server rather than through the PHP mail() function. Please ensure you use SMTP authentication with an e-mail account set up on your web hosting account.
For more information, please see the PEAR PHP page on using the Mail class.