since I updated to mac OS Big Sur 11.5.1 I have stopped receiving emails on my Mac!!!!!

Depending on the email service provider, you may need to use a new device password. Try logging in directly to the service online to generate a new device password.

I can't say why this is happening, but the first thing to do is to go into your email app and check all of your settings and passwords. Most likely they have gotten messed up. This information is normally available in Preferences.

I faced the same issue. All the accounts went offline. I tried to make the accounts go online, but it didn't work. So, I removed the accounts and added them again. It started working again.