Send-a-Friend Link

Bear in mind that if you introduce this feature you will also have to implement measures to prevent your site from being used as proxy for automated-spamming software.

E.g prevent third parties from receiving :
--------
Hi {NAME OF RECIPIENT},

You're friend {SPAMMER} thought you'd be interested in this
{ITEM}. Buy it at {URL_TO_ITEM}.

Here's the Personal Message from {SPAMMER}:

Buy viagra at www.example.com
-----

There are two reasons why you don't want to send spam on behalf of others:
1. Spam is bad
2. Email network's Bayesian spam filters will learn that your site sends spam and may block any genuine emails that derive from your website in the future - on a group level.

Prevention:
* keep an eye on the emails that you send out so you notice if you are sending huge amounts
* add a captcha system
* spam filter messages BEFORE you snd them out

Can you tell more about the SPAM filter before outbound messages? Is there a specific product or just a configuration on any SPAM filtering (inbound) product?

We use Symantec enterprise edition.

Also keep in mind that even if spammers aren't abusing your form (either using header injection or by entering a spam message), the tell-a-friend message itself may run foul of anti-spam legislation.

Arnoud Engelfriet explains how to steer clear of Dutch anti-spam law here. (Executive summary: 1 - do not offer the sender a reward for telling a friend, 2 - do not try to hide who the friend is, and 3 - let the friend preview the entire message before sending it.)