Sendmail not sending to all adresses ?

i have a quite specia problem.

I have a script to send a password to users who registered to be able to give comments on newsposts.


Now, it works fine with @hotmail.com and @yahoo.com . However on other adresses (pandora.be , skynet.be, zzn.com) it does not work. So are reporting several other users.

On the forum on the site the same thing happens for resending a password to users. So i suppose it is not a script-problem.

i am using /usr/lib/sendmail
i also tried /usr/sbin/sendmail

does anyone have any idea ?
the serveradmin doesn't know so maybe a suggestion ?


thx very much