As of March 2020, Yahoo! Mail accounts no longer offer the "Allow apps that use less secure sign in" option. In order to connect to a Yahoo! Mail account with Aid4Mail, an "App Password" needs to be generated.
To create an App Password, login to your Yahoo! 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
Do you have Yahoo!'s 2-Step verification set up? Click here for details on Yahoo!'s 2-step verification
The IMAP settings screen in Aid4Mail should look like the image below to connect with a Yahoo! Mail account.
Note: Insert your own username and password but leave the "Namespace" field blank.
Should you require any assistance, please don't hesitate to contact the help desk.
If you would like to buy an Aid4Mail license please visit the Aid4Mail website.