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.

BTW, some mail servers now implement a technique whereby they reject an email the first time around. This is because compliant mail servers will try again later, while non-compliant ones (aka spambots) usually do not.

Go read up on current antispam techniques. You may need to send an email twice (with a timegap in between), or use a seperate real mail server to forward what you're trying to send (and let it worry about resending instead).