SMTP Header problem

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.2
-- PHP version -- : 5.5.43
-- Browser(s) name and version -- : FF & Chrome - updated

I've been in chat with Microsoft Admins and HostGator Admins. Both have indicated that they may be an issue with SMTP formatting in the headers of emails being sent by HikaShop. However, I can't believe this is the issue as the message I am getting from Hotmail is this:

Diagnostic-Code: smtp; 550 SC-001 (BAY004-MC4F46) Unfortunately, messages from 162.144.146.144 weren't sent. Please contact your Internet service provider since part of their network is on our block list.