Undelivered emails - System notifications are delivered however Email address confirmation and password reset not delivered

Dear all,

I have another issue (I have several). I receive notifications and other
general system updates thorugh email. When I sign up from the homepage, I
do not receive email to confirm my address. Also, password resets are not
delivered. Strangely, I receive the notification that I created a new
project however, I do not receive the email to confirm my email address.
What could be causing this?