As the title, does anyone have any idea of why this would be true?
I simply output to mp4 with default settings and it plays locally but
not when sent to others.
What are they trying to use for opening mp4 files on their end?
Player uses ffmpeg to read and encode mp4 files. FFmpeg and/or any application that uses it should do fine. For example VLC for PC, or even the built-in media player in Windows. For mac - they can use quick time.
Edit: player always encodes mp4 with H-264 encoding (the most popular one) if that helps in any way.
I use VLC so no issues. I sent the guy your response, so it’s up to him to sort out now 
He should know to at least experiment but y’know…some people! 