Most Yahoo! accounts require you to create an app password when connecting to an account via a third party application such as Aid4Mail.
Also If you have two-step verification turned on, then you will need to create an App Password.
To create an App Password, login to your Yahoo! Mail account and go to the "Account Security" section under "Account Options" and click on "Generate app password", as shown here:
After you have created the App Password, copy it and use it in Aid4Mail as the "Password" for your Yahoo! Mail account.
For further assistance, please refer to the following Yahoo! help article: Generate third-party app passwords