Mailbox full error even though it's not full
https://answers.microsoft.com/en-us/outlook_com/forum/all/remote-server-returned-554-520/1ab850a3-339f-4504-a02e-a5de2bbe3a6f
Try enabling the users archive folder and then expanding it
- Connect to Exchange Online Powershell
- Use this command: Enable-Mailbox <user mailbox> -Archive
- Then this command: Enable-Mailbox <user mailbox> -AutoExpandingArchive
https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving