When you first launch Postbox, it should take you to the New Account Setup page. If it does not, or you are adding an additional account, click on File > New >Mail Account
Provide your Name and Email address, then click Manual Setup
Enter your server information,
Incoming Server Name : imap.mail.yahoo.com
Type : IMAP
Port : 993 or 143
Security : SSL/TLS
Outgoing Server Name : smtp.mail.yahoo.com
Port : 25 or 465
Security : SSL/TLS
Then click on Finish
Type your password and press OK
Now you can check your Yahoo account using Postbox
