I can't take credit for this, but someone on my team (Juan Ngomo) shared this with me recently.
When adding a superscript for items like ©, ®, th, st the email editor preview displays the superscript correctly but when the email is received, a space is added between the descriptive word or number and the superscript.

Few things can be done to solve this issue
- Highlight the paragraph where the superscript was added & change it to "Double Space" as the image below (1)
- Open the editor and edit the HTML in the WYSIWYG editor. Change your <div> tags to <p> tags in the html code which is the same thing as the point 1.
- Also by surrounding the text with <span>31st</span> you can force the entire element to be rendered as one continuous line, this is also done in the editor
(1)

Hope this is helpful!!