Folders with JPEGs were around 5 Mb.
At this size, the image size option on the bottom right disappears, and Mail compresses the message, thus ruining the images.
There is no alert that it's doing this, which is insane. I'd rather have a message not sent than images arriving at the other side compressed and ruined, which can cause a lot of damage to a business.
There has to be an alert, but there isn't one.