Its possible but why would you want to do that? it would break things like forums etc.
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Like Steven said, it's a bad idea to completely turn it off - however, it's possible to limit the dangers of the abuse quite a bit. There are patches that you could install to stop injection into the mail() command from legitamate variables. However they still cause problems with some boards. Quite a few boards now a days have pop mail facilities, and you can use that much more easily.
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Thank you all, my first choice is not to disable it, just secure it. I once worked with a server and aparently mail() couldn't send e-mail to domains which where not in the server, and messages to internal accounts arrived, is this possible?