All mailboxes are empty since upgrading my operating system

Hey there East422,

Let's take a closer look together to help find your missing emails from your mailboxes.

Here are some steps we recommend:

1. The first step we recommend is to check the websites for each email account you have set up currently in Mail. If the same emails are missing from all of your mailboxes on those websites, this is an issue from the email provider(s) side of things.
2. If the emails aren't missing when you check on those websites, we recommend a restart. Check out the section 'Restart' here: Log out, sleep, wake, restart, or shut down your Mac
3. We also see you're using macOS 11.2, but the latest version is macOS 11.2.3. You can verify the version number installed by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’. Here's how to first Back up your Mac with Time Machine, and afterwards, here's How to update the software on your Mac.
4. If you aren't receiving any new emails in your Inbox, we also recommend taking the steps here: If you can’t send or receive email on your Mac

Take care!