Yahoo's mailboxes are full

552 5.2.2 This message could not be delivered because the recipient’s mailbox is full. Please try again later or contact the recipient directly. See https://senders.yahooinc.com/smtp-error-codes#mailbox-full for more information

Yahoo used to have huge mailbox limits, from when they were competing with gmail on that checkbox feature. A terabyte or more.

They’ve recently reduced them to 20 GB - which is a lot less so there will have been a lot of users who were suddenly over the new quota.

Yahoo notified users about these changes and explicitly notified any users who are over the new quota, and they gave them a grace period to clean up their mailboxes.

That grace period seems to have expired, and so there’s been a recent spike of rejections due to mailboxes being full.

Those users with full mailboxes won’t be receiving any new mail, and will have had plenty of notifications from Yahoo to clean things up. They haven’t. It’s fairly safe to assume that those mailboxes have been abandoned, probably for years. Or decades.

So the spike in rejections doesn’t mean anything about your mail programme other than that you’re sending to people who don’t exist any more. Just suppress those recipients when you see this hard bounce.

And if you’re seeing a lot of these mailbox full rejections, maybe take a look at how much engagement you’re seeing from your recipients. Maybe some others don’t exist either.