In your yahoo account settings under "Account Security", you will need to make sure that "Allow apps that use less secure sign in" is turned on, like this: 
Do you have Yahoo!'s 2-Step verification set up? Details about Yahoo!'s 2-step verification:
https://help.yahoo.com/kb/SLN5013.html
If so, you'll need to use an "app-specific password" for Aid4Mail. Here's the details:
https://help.yahoo.com/kb/generate-separate-password-sln15241.html
Please leave the "Namespace" field blank.
The IMAP settings screen should look like this (with your user name and password of course!):
If you experience any issues, don't hesitate to contact us.
If you would like to buy an Aid4Mail license, visit the Aid4Mail website.