My answer would be:
"Please make sure that your mail server is configured to handle multi-part (HTML and text) mail messages."
I think that I did ask what specific configuration is required by HikaShop once or twice between the explanations of what the "server does not like" and Joomla vs. HikaShop message sending but I lost this answer in the details.
Here's what the current HikaShop documentation says:
Multiple Part : Should HikaShop send multiple part emails? If so, HikaShop will send the text and the html version in the email so that depending on the recipient server, the appropriate version will be opened.
Maybe this would be a good place to say "Requires website's email server be configured to support sending multiple part messages."
Raise your hands if you are implementing an online store and did not know that there was such a setting? Oh... maybe it's just me.
Cultural context: meant to be funny and just a little frustrated.
And yes, I agree this is not a bug in the code.