During email verification we can tell if an email address exists. We do this with a so called SMTP connection. We create a connection with the recipients' servers and we initiate an SMTP connection. This is the same connection that is used for email sending. The only difference is that we do not send an email.
Yahoo has recently disabled a large number of accounts. The email accounts still exist hence we detect as OK, but they send a bounce message due to account inactivity. When verifying those accounts we get an OK response, but when an actual email is sent they send a bounce message that the account is disabled.
This affects only a small % of yahoo ok emails, but be aware this can happen.
The only way to get this response from Yahoo is by sending those accounts an email. We never send an email during email verification as we would be spamming your subscribers.
When can I use Yahoo OK emails?
If your subscribers are real subscribers and they have recently subscribed you can use them.
When should I not use Yahoo emails?
If you are using an old database or collected emails form the internet it is better not to use Yahoo emails.