Will you be able to describe your situation in detail for better troubleshooting?
1.What is the user's mailbox type? Is it a shared mailbox or a public folder?
2.Have you received an NDR? Can you attach a screenshot?
3.Are there mail flow rules that affect mail delivery?
If there is any other useful information, you can also add it or attach a screenshot at any time.
Based on the error code you provided, here are the common workarounds. NDR error 554 5.2.2 mailbox full when sending email to mail-enabled public folders - Exchange | Microsoft Learn
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