New users not receiving passwords

I setup WP in February and several emails were sent to new users.. but now, after setting up more users.. no email is being sent, even though they get the “created successfully, password was sent” message. I even receive the new user notification to my email account.. but the user does not receive email from WP. I have checked my database functions and all the WP files on the server. Nothing appears to have been edited since the day I installed WP.

Any help will be greatly appreciated.