email problem after upgrade

I have upgraded to 1.27 and everything seems to be working fine, I just have one problem. The email is working on the inquiry and also when you place an order. I receive both corresponding mails, the only one that is not working is when you sign-up as a new customer, the mail confirming the sign-up is not received. Could you help me see where is the problem? If it helps my provider is in UNIX, and I work in a Mac. Thank you in advance for your help.