Mail.app reporting not enough space with 263GB free

A few days ago mail started reporting that it could not save information because there was not enough space available in my home folder. My iMac has 263GB free, so I don't understand why it would report this. I've tried restarting and running disk first aid from recovery mode. Nothing has helped.

I've also noticed other minor issues that could be related to this. Things like screenshots not saving and wallpaper/screensaver settings changed to default on restart.