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That has been an ongoing problem for many years, some of the answers telling you what you need to do for SbcGlobal email having that problem which was taken over by AT&T years back, other varying answers.
However do keep in mind that Outlook is not a Yahoo product so we can not help you with it. You might try the solution that worked for me. I used Outlook Express and liked it until they chosed it So I switched to Outllook but it refquently had problems they claimed were not theirs. So I quit using them.
You might want to look here. https://support.office.com/en-us/article/Get-help-with-Outlook-com-40676ad0-c831-45ac-a023-5be633be7...
Chances are they will eventually tell you it is Yahoo's problem which is untrue and at that point you can do what I did and remove them from your system.
Found the solution, not easy! Two things were necessary, enabling 2-factor authentication on Yahoo (not easy in itself) and then GENERATING A SPECIAL PASSWORD for Outlook. In a browser, log in to your Yahoo mail account, then go to Account Security, which is NOT found under the Email Settings. It is found by clicking on your picture or person silhouette in the upper right corner and choosing your account and then choosing Account Security. Scroll down to "Two-Step Verification" and turn the button on. It will ask you for your mobile number and will send you a code you have to enter on the page. However, YOU MUST ENTER YOUR PHONE number without punctuation such as () and -. Just digits. Selecting it from the drop-down box won't work if it has punctuation in it, you have to manually enter it. Then, after you've received the code and entered it, you will have an opportunity to generate passwords for apps and devices that don't work with the 2-step verification. You can also get to that password generator directly from the Account Security page, which is how you will do it henceforth, after having enabled 2-step verification. You go to that password generator and select Outlook Desktop as the app. It will prompt you as to whether you want to generate a password for Outlook Android, which is not what you chose, but don't worry, just generate the password anyway. Now, when Outlook asks you for a password, use the one that was generated (16 lower case letters, don't put in any spaces) instead of your actual Yahoo mail password.
There are some complicated suggestions here, but I was able to fix the same problem by simply Allowing Less Secure Sign-in. That saved a bunch of other steps and took less than a minute from the MAIN screen in Yahoo, not the email screen. Try this first!
While you are using that setting that also makes your account more vunerable by letting apps access your account using a less secure method.
Ways to securely access Yahoo Mail - https://help.yahoo.com/kb/new-mail-for-desktop/sln27791.html
Generate third-party app passwords - https://help.yahoo.com/kb/mail-for-desktop/sln15241.html
Outlook is not a Yahoo product. You will need to contact them if you need help.
Let me rephrase my first reply above. The solution offered by the user "Outland" and time stamped as "02-19-2018 06:18 PM" on page 2 worked for me and solved my problem. Thank you very much Outland. You saved me from wasting more time looking for a solution.
I have a fix which is rather simple.
On your phone (I use an Android device) and launch Yahoo Mail
Click the top left corner (the 3 bars)
Click ‘Manage Accounts’
Click ‘Account info’
Click ‘Account Key’
Click ‘Get an app password’
An App password 16 letters would show
Use this password on your Outlook Mail on your computer instead of your normal Outlook password.
This would fix the Yahoo Mail password issue on MS Outlook mail.
The reason that I use a phone instead of a computer is that I could easily copy the 16 letter password to my Outlook Mail on my computer.
After a month or longer, the Outlook would ask for Yahoo Mail password again. Just repeat the steps and it will be fixed again.