You are receiving emails from Joomla Core, not even from Virtuemart Component
CleanMart is a template for Virtuemart Component, not even for Joomla.
So from this standpoint, no need to mention, but i am going to say it anyway:
Sending or Failing to Send Emails upon user registration has absolutely nothing to do with CleanMart.
We don't provide free support if the issue is not related to our products.
Use google.com and check forum.joomla.org, i am sure you will find many topics about this issue. Because it is a common bug for joomla websites.
Topic is closed.
Regards.
Support