We are using a custom email message, and our system is multilingual, so it is based on a language override.
This is what part of the message looks like in English when received by Hotmail. I have highlighted weird symbol insertions in red:
And this is what the Japanese version looks like:
We have set it so that we, as admins, receive the exact same email as is sent to the user and our emails do NOT contain these weird inserted symbols. It is ONLY the hotmail (or outlook) emails that have them. So we are confident that our email is formatted properly. Also, we have hundreds of new users every month and only those with Microsoft based emails are having this trouble, so we are confident that our email is formatted properly.