registration emails not sent to new users

Hi, I try to create new wpmu, in this domain the registration email not sent. After not finding solution I delete the account and start from the begining, I check that regestration email with the same domain before install the MU and it work then install the mu and the email system work for recover lost passward but not for new users registration. there is no plugin or theme on that account. If someone have an idea what happen I will be very happy to know, Thanks