Yahoo Mail POP import discontinued, use IMAP instead
If you're using POP (Post Office Protocol) import to pull emails from another email account into Yahoo Mail, we encourage you to connect your external account via IMAP import. After March 31, 2017, we will no longer support POP import on desktop, as we're retiring the legacy system that supports this protocol. Any emails previously imported into Yahoo Mail will remain.
IMAP is a better, more modern protocol and experience that lets you connect email accounts like Google, Outlook, Office 365, Hotmail, AOL, and others using multiple mailbox management. You’ll be able to access every email and folder from your other accounts in Yahoo Mail desktop and the mobile app!
If you're unable to add your account using IMAP, many third-party email accounts let you turn on "Mail Forwarding," which will allow you to forward your email from that account to your Yahoo Mail. You can then add the third-party account in Yahoo Mail as a send-only address, allowing you to send from the third-party account in Yahoo Mail.