Yahoo Mail not working with apple mail app version: 13.4 MAC Catalina OS 10.15.7

Mac's Mail App is not as modern as iOS's Mail app, Many eMail providers now require a separate App Password different from your eMail password…

Yahoo has gone to this too...

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

Yahoo App password…

Generate and manage third-party app passwords

Some older, third-party email apps (that do not use our Yahoo branded sign in page) require you to enter a single password for login credentials. To access your Yahoo Mail account on these apps, you'll need to generate and use an app password. An app password is a long, randomly generated code that gives a non-Yahoo app permission to access your Yahoo account. You’ll only need to provide this code once to sign in to your third-party email app.

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

https://help.yahoo.com/kb/SLN4075.html

Don't forget the port numbers.

If there isn't a separate port field, it usually is inserted with a semicolon between the port and server name -

e.g.:

smtp.mail.yahoo.com:587

Get Yahoo's IMAP settings, and configure Mail using Other... account type, and choosing IMAP.

Older OSes do not have the most current defacto settings for Yahoo.