problem with login/register new account using wordpress MS website

i am new in WP,,,
i just installed wordpress multisite and started to create a membership website using one theme,every thing is going ok but for register new account user(sign up): WordPress does not send any email conformation to this new user and showing this message at sign up page “

before you can start using your new username, you must activate it,Check your inbox at the mail you have entered and click the link given

“!!!
when i check up the database every thing is happened the mail has sent but actually the new user cant receive any thing so he can not get his activated password thats will make him not able t login
any help it will be really appreciated
thank .