Syncing mail between iMac and iPhone

Before I upgraded to Big Sur, my mail on iPhone and iMac was perfect. If I checked mail on the iPhone during the day, and sent to trash that which I did not want, I would find that they were also in trash on the iMac when I got home in the evening. This also worked in reverse. Now, whatever I send email to trash on one device is still on the other device and I have to manually send them to trash as well. them as well. I am not sure what happened and I would appreciate any and all help. Thanks.