Outlook Business email stuck in Outbox - Error: 554 5.2.2 Mailbox full⁠

Hello Alex Cloro
Thank you for post question to Microsoft Q&A Forum.
According to your question, please follow these steps:
1.Go to Outlook on the web.
2.Click on Settings at the top right corner > Account > Storage (it should look the same as this screenshot below).
Reference: https://support.microsoft.com/en-us/outlook/mailbox-storage-limits-in-outlook

3.Clear/Empty the specific folder to clear space for your mailbox.

If all of your folders are full and you do not prefer to delete any items, please proceed to enable Online Archive for your mailbox via Exchange Admin Center then set up retention policy to move your old emails into this folder
If you are not the administrator, please reach out to your IT department and seek assistance from them to set up the retention policy.
Please let me know if you have any further questions.

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