I recently upgraded to wordpress 3.1.3 and also upgraded php to 5.3.6
To begin with i thought the problem was with sending of email from the web server in general. Using my hotmail and work account I wasn't receiving any email from wordpress, new registrations or lost passwords. A colleague with a gmail account did receive her registration email and i have just created myself a gmail account and i immediately received my registration email. I have googled this issue intensively today and haven't found many people reporting this issue. Is this a bug or a configuration issue? Can anyone point me in the right direction?