Hi,
HikaShop emails have HTML in them while normal Joomla emails don't. HTML allows for a nice layout for the emails.
If you have the Business edition, you can edit the emails via the menu System>Emails and turn off the "HTML" setting of the emails to see if that's the problem.
If so, then I would recommend to contact your hosting company to see why they refuse HTML emails and not non HTML emails.
Otherwise, it's something else with the Emails settings of the HikaShop configuration. It could be that you turned on the "embed images" setting, or you might need to change the charset or the encoding based on what your hosting company prefers.
Or maybe it comes from the "from name" which should be left empty, or the bounce email address which should be from the same domain name as the "from email", etc.
There are a lot of potential causes which depends on what email server your hosting company is using and how they configured it.
That's why I keep recommending to ask them. Otherwise, we're just shooting in the dark.