UPDATE: Spike in Yahoo unknown users
I still don’t have any solid information on the cause of the Yahoo bounces. I do know that folks inside Yahoo are looking into the issue.
However, multiple people (including my clients) are reporting that the addresses that are bouncing have very recent click and open activity. Other reports say these addresses deliver on a resend.
It looks like my advice yesterday was incorrect. I’m currently telling clients to continue mailing addresses for the time being.