Mail glitches on my Mac
Mail on my iMac is behaving very unfunny. When I delete the trashed letters mail shuts down and now the trashed email keeps coming back to my inbox. I'm running version 11.5 (3445.9.1). I'm also running High Sierra 10.13.6. I can upgrade but never like to because things change so much it's a PITA to figure out new stuff. Do I have to upgrade to make Mail behave? Thanks
iMac 27", macOS 10.13