How to Troubleshoot a Mailbox Full Error in Microsoft 365
Question details
The sender is receiving a non-delivery report (NDR) stating that the recipient's mailbox is full, even though the recipient is still able to receive some other messages.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending an email to an internal or external recipient and having the message bounce back with a full storage error.
- Observed behavior
- A non-delivery report is generated with diagnostic server details (e.g., AM8P193MB0802.EURP193.PROD.OUTLOOK.COM) indicating the recipient has exhausted their mailbox capacity.
Keep a copy of the complete Non-Delivery Report (NDR) handy, as the diagnostic server information contained within it will be necessary for administrators or support teams to investigate the exact cause of the bounce.
Check Storage Usage in Outlook on the Web
Ask the recipient to log into their webmail to verify their actual storage capacity and clear out unnecessary emails if their limit has been reached.
Even if a user can occasionally receive small text emails, a nearly full mailbox will reject messages with attachments. Verifying the current storage status is the first troubleshooting step.
Have the recipient navigate to outlook.office.com and sign in with their Microsoft 365 credentials.
Click the gear icon located in the top right corner of the screen to open the Settings menu.
Go to General, then select Storage. This page will display a bar chart of current mailbox usage and the maximum allowed capacity.
Review Mailbox Quotas and Message Trace (For Admins)
Microsoft 365 administrators should investigate the recipient's mailbox quotas and use the message trace tool to determine why the delivery failed.
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Frequently Asked Questions
Why does the Non-Delivery Report say the mailbox is full when the recipient can still receive some messages?
This can happen if the recipient's mailbox is hovering right at the storage quota limit. Small text-only emails might still slip through, while larger messages (especially those with attachments) will be rejected, triggering the 'Mailbox Full' error.
Where do I find the diagnostic server information in a bounce-back email?
The diagnostic server details (such as AM8P193MB0802.EURP193.PROD.OUTLOOK.COM) are typically located near the bottom of the Non-Delivery Report (NDR) email, under a section labeled 'Diagnostic information for administrators'.
How can an administrator increase a user's mailbox storage limit in Microsoft 365?
Administrators can increase mailbox limits by upgrading the user's Microsoft 365 license (e.g., from Business Basic to Business Standard/Premium), enabling Exchange Online Archiving to move older emails, or adjusting custom quota limits via Exchange Online PowerShell.
