Thank you for reaching out to us. This is a known issue with Outlook, and we're continuing to work with Microsoft to resolve the coding problems.
The only guaranteed workaround for this issue at the moment is inserting a View As Webpage link. When your contacts click this link it will display the email in its absolute purest form, unadulterated by any email client's coding or IT firewalls.
I understand it's not the most ideal practice, but as this seems isolated to one type of email client and not to any specific email service provider, this is the most effective workaround available.
If you have any additional questions or concerns, feel free to reply to this thread and we would be happy to assist.