Instances occur when you have to struggle with Yahoo Mail not syncing with Mac. In such a case, you need to ensure that you're using the most updated version of Mac. You can also visit Yahoo Mail and sign in to your account to compose an email and send it to your own Yahoo address to check if Yahoo is functioning properly.
Also Read:-
disable incognito mode chrome iphone
facebook search bar not working
hp printer won't print color
canon printer won't print color