Glad to see some options like Thunderbird exist but you must use the older version that is compatible with MacOS 10.11.6. Another free client is Mailspring. It is full-featured, updated regularly and works great with Yahoo, iCloud, and any IMAP mail.
I did, however, find a workaround. Changing your Yahoo account access in Apple Mail from IMAP to POP restores the ability to receive mail again. But do understand this can become very confusing if you continue accessing this account via IMAP on other devices. For that reason, I don't recommend this approach but if you want to give it a try, here is how:
First you’ll need to log in to your Yahoo Mail account via a web browser and generate an “App Password” for use with Mail. Click on ‘your username’ and then ‘Account Info.’ You may be asked to login again. Do so and then click on ‘Account Security.’ Scroll down to ‘Account Access’ and further below that you will see a section titled “This is how you sign in now.” Scroll further down to “Other ways to sign in.” Click on “Generate and manage app passwords.” A small window will pop up titled “Generate an app password.” It asks you to “Enter your app’s name.” Type “Mail” without the quotes. Then click the ‘Generate password’ button. Click the ‘Copy’ button to copy the password. Leave that pop up window showing the generated password open until you are done below, just in case you need to refer back to it.
Now launch Mail. Click on ‘Mail’ in menubar and choose ‘Accounts…’ This will open ‘Internet Accounts’ (in System Preferences) where you’ll need to DELETE your old Yahoo IMAP account from the list presented. Click on said account and then the minus button at the bottom of the list to delete it. Close window.
Back in Mail, click on ‘Mail’ in the menubar choose ‘Preferences…’ this time. Now click on the ‘Accounts’ tab. Click the plus button at the bottom of the list which will open a new window that says “Choose a Mail account provider…” Do NOT click on ‘YAHOO!’ Instead, click on ‘Other Mail Account…’ An ‘Add a Mail account’ window appears. Enter your Yahoo Mail ‘Name’ and ‘Email Address’ but use the newly generated “App Password” you copied earlier. Click ‘Sign In’ button.
It may take a while for anything else to happen. But wait until you get the expanded window and error message in red that says “Unable to verify account name or password.” Now change the ‘Account Type’ to POP. Change the ‘Incoming Mail Server’ to pop.mail.yahoo.com. Change or confirm the ‘Outgoing Mail Server’ is set to smtp.mail.yahoo.com. (Don’t worry if the ‘User Name’ has been changed to “Automatic” and greyed out.) Now click ’Sign In’ button again. Within 30 seconds the window will close and the account will be successfully added to your Accounts list. You can now receive your Yahoo mail again!
