Option 1 - Enable less secure access
Sign in to your Yahoo account.
Click on your avatar and pick Account Info.
On the profile page pick Account Security tab.
Scroll the page down and click on Allow apps that use less secure sign in.
Enable this option to let StealthMail applications to access and decrypt secure emails for you. Please note: Yahoo may ask you to solve puzzle as additional validation check.
At this point you may try to add your email account again.
After re-entering your password your account should be added successfully.
If not, try Option 2.
Option 2 - Generate App Password
On the profile page pick Account Security tab. (SeeOption 1to see how to get to Account Security tab)
Scroll the page down and click on Generate app password.
In the appeared dialog pick Other App.
Enter StealthMail in the text input and press Generate button (you may enter any other application name).
You will get a generated password. Copy and save it for later use.
Enter your email account name and saved password.
In a while your Email account should be successfully added.