Why does a valid email address bounce?

Why does a valid email address bounce?

This is biting our app as well. It is not resolved at least according to chat support a couple of nights ago.
See known issue

Yahoo/AOL Email Rate Limiting

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Issue Number: 2505632

Issue Status: In Progress

Date Reported:
08/12/2021

Estimated Resolution Date: 9/2/2021

Summary:
Update - Wednesday 9/15/2021 9:50am MST: We are working closely with the postmasters to get some relief on some of the limiting. Another update will be made when more information is available. We are currently seeing rate limiting on our email network related to any mail sent to Yahoo or AOL related domains. What this means is that mail to any recipient on the Yahoo/AOL network can see the delivery delayed for 30 minutes or more. Also, you could see an increase number of bounced mail in your app from mail sent to the related domains. In most cases we are seeing that the mail does deliver on successive retries however there are also some pieces of mail that do not deliver in the 12-hour time frame in which we make the retry attempts. This mail will not be sent after this time frame expires. Any address that is affected by the rate limiting will NOT see any changes to the email status so they will stay in a marketable status. We are working with the Yahoo postmaster to get the network back on track to get this resolved. We have already identified some problem users and working with them to fix the issues they are seeing as well. The IPs that where seeing the biggest impact have been removed from rotation to give our users the best chance to get their mail delivered as soon as possible.

Alternative Solution:
There is currently no workaround for this issue, but we are working to resolve it as soon as possible.

Product Line: Keap & Max Classic