Verizon.net email account no longer recognized by iMac with Big Sur

I suggest starting with double-checking that you have the correct email server information configured for your Verizon email account in Apple Mail. To do so, try using this Apple Support site: Mail Settings Lookup - Apple Support

If the server settings are correct, do you enable two-factor authentication with your Verizon email account? Can you access your email account via a web browser successfully?

You may find the following support article helpful: Troubleshoot email problems in Mail on Mac - Apple Support

It's that email we have been getting for the past 6 months that if you use a 3rd party (non-AOL) mail client, you have to use a one-time password.

I enabled two factor so I have to enter a code from an authenticator app (I use Duo but you could use Google Authenticator as well), then I went to the setting to create a one-time password. Reconnect with that password in Mail. It didn't immediately fix it. Next time I launched everything is OK.

Log into your AOL account using a web browser.

In the upper right, click on “Options”

Go to “Account Info”

Click on “Account Security”

At the bottom of the page, “Manage App Passwords”

At the pop up window, select “Mac Mail”, then “Generate”

Select and copy the key created...

Go to Apple Mail

There are THREE places to paste the key, the third is under SMTP sub-window...

Save and quit Mail, reopen.

Check Connection Doctor to see if AOL mail is being logged into correctly, both incoming and outgoing...