A five second MP4 video clip recorded with Android’s default Camera app has a resolution of 1920x1080 and a size of 15 MB.
If this video is passed to Delta Chat, it is converted to 360x640 of size 1 MB.
So far so good.
Sent to an iPhone or iPad, it does not play. You click on ‘start’, but nothing happens.
Sent back to Delta Chat on Android, it plays perfectly.
Apparently, MP4 is not an ideal format for platform interoperability.