So the issue is actually that your email server is not able to send the email with the new email address that you had set, and not that you couldn't change the email address in HikaShop.
In that case, I would recommend to contact your hosting company to see why it doesn't work with that new email address. They must have specific requirements for the email settings of the HikaShop/Joomla configuration.
As I said in my previous message, HikaShop only uses the email addresses configured in the Emails settings of the HikaShop configuration and the email addresses in the settings of each email notification that is editable via the menu System>Emails. There is no other place, and no bug on that.