php mail()

Why is it that Gmail gets mail from mail() fine, but it never goes through to other mail services like hotmail and yahoo?

I've tried using just "From:"; I tried using some other headers (the mime type and character encoding), but that didn't work; I looked in the php manual and added just about every extra header I could find, tried it, and it still didn't work.

The only antispam technique that works is driving to a spammer's home and bashing his skull in.