The same problem occurs not only in Naver mail where my account is connected but also in Google mail.
The phrase below is common. Is there any solution or other means of contact?

552 5.1.1 <support@itch.io> Mailbox delivery failure policy error

We had a temporary downtime with our support email address yesterday for a few hours. The issue should be resolved now. You will need to send your request again if you did not receive a receipt email.

Gonna close this topic since we have another open about it.