This week there was a reported uptick in user unknown responses for verizon email addresses. The specific response folks were seeing was:
554 delivery error: dd email@example.com is no longer valid. [-20] -mta4047.aol.mail.bf1.yahoo.com
This appears to be a problem with their require-recipient-valid-since header checking. They are aware and are working on a fix.
What this means for senders is if you’re seeing Verizon addresses hard bounce, then you shouldn’t necessarily drop them from your list. They are likely still valid. You can continue to retry mail to those addresses. This does seem to have just affected the verizon.net addresses, not other addresses across the properties.