Being able to add your e-mail on your phone is very important. I personally use my e-mail for the most part so I can understand your concern. Are you able to add other e-mail accounts on your phone without any issues? Are you using the native e-mail app or a third party app to add this account?
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IDK which model you are using but this page has a few ideas:
One says remove any app password you might have set up for Yahoo, another swears Yahoo and BB don't work well together since a few internal staff changes recently, and another says go Gmail, don't look back, even more say they created another email on Yahoo which might point to password issues since afterwards, the email worked and others say create a new gmail account and forward the Yahoo to that address.
Hello, I experienced the same problem on my BlackBerry Classic. I have two different email addressed with @yahoo.com and I changed the passwords on both accounts and restarted the phone - entered the new password(s) and the issue has gone away. I manage more than a handful of emails on my phone and the above trick has always worked with any email account.
If the above does not work for you please let me know. There are other options possible.