email - Error sending special characters on mail function (php)

I'm using the mail function on php to send a confirmation
email. It actually works, but just for some mails. For example, if you recieve that on a
gmail it's fine, but on a college mail it appears ? instead of special
characters.
The problem is that that mail includes a validation link,which
looks like www.myweb.com/confirmation.php?passkey=(passkey) and if the mail and the
special characters aren't send properly this link is also wrong (the = doesn't
appear).

I've already tried writting

iconv_set_encoding("internal_encoding",
            "UTF-8");

on top of the mail
function, but it doesn't work. What can it be?

Thanks for
your help!