What if the recipient claims their mailbox is NOT full, but I'm still getting the '552 5.2.0' error?

If the recipient insists their mailbox isn't full, there might be an issue with how their email provider is handling storage. Try sending the email again a few times, and also confirm the recipient's email address is correct. If the problem persists, advise the recipient to contact their email provider's support for troubleshooting, as it might involve internal technical errors on their end.