Mail cannot save information about your mailboxes because there isn’t enough space in your home folder.

Last week my MacBook Air started doing this. I open Apple Mail, I can receive some emails, then after approximately 3 minutes, I get this message "Mail cannot save information about your mailboxes because there isn’t enough space in your home folder.", the only choice is to "Quit" Apple mail. I Check my settings Storage, it says I have 837 GB of "System Data". This started after downloading IOS 15.1.