Registration email not being sent

Turn off the "Multiple part" option of the HikaShop configuration and try again.
If that doesn't help, please contact your hosting company with a screenshot of the Emails options of your joomla configuration and a screenshot of the Emails options of your HikaShop configuration and ask them what is the problem with the emails not being sent by the email server. They should be able to tell you.

Thanks for the quick reply....

When you say "Turn off the "Multiple part" option of the HikaShop configuration and try again."

Where is the "Multiple part" option ?

I have found where to "Turn off the "Multiple part" option of the HikaShop configuration, it is the last option in

Hikashop --- Configuration --- Email

It did not solve the problem so I am sending screenshots of my Hikashop and Joomla email configurations to my hosting provider to see what they can recommend.