google or yahoo?

which sites are you talking about? I have never come across any such website... some sites could have their own databases or some features which will not work with yahoo-mail.. sometime it is an error with the box where you are typing the email address.. you could just contact the site admin about this..

i've notice that too yahoo not allowing alot of sites i don't know why.

I really do not know why.. But most likely this is to prevent spam registrations and such. And ofcourse to prevent abuse of registration procedures.