Email formatting not working in hotmail & Outlook for Mac

After sending a newsletter out to 500+ subscribers, one came back with a completely broken email.

This was using a customised template design. I tested the email again using one of Mailpoet’s own templates but got the same problem again.

All over the email are ‘=’, images dont download (even when clicking download images) and HTML styles show and look like they don’t have a closing ‘>’.

The email looks fine in Outlook for windows, Gmail, Mac mail and iOS.