Yea so people have been spamming my Guestbook like crazy. I even implemented a check based on time of previous post and IP. You would have to wait 10 minutes to post another guestbook entry.
I also had to striptags() to the entries. Why post useless spam? At least post something that I might actually click on. So I've removed the ability to add to the guest book.
One of my tasks at work is to go through the daily spam which is roughly 1200 email messages a day that get caught by our spam filters. Roughly 5% if not less is actual email that gets caught for dumb things like when people send in email with MAC addresses and have an octet with xxx or something.
To be able to create the perfect Spam Filter would be cool and make alot of money. Though spam gets harder and harder to filter. People spell things in all sorts of different ways. C12AL37IS, PH12AR38M&uaAC83uY etc are some of the examples. Though as one can see, the upper-case letters are what spells out the words.