How to Fix Office 365 Mailbox Full Issue?

Issue: If you’re seeing a “Your Office 365 Mailbox is Full” warning, it means you’ve hit your storage limit. When this happens, you won’t be able to send or receive emails until you free up space or upgrade your plan.

This guide will help you understand why this happens, how to check your storage, and how to fix it step by step.

How Much Storage Do You Get in Office 365?

Mailbox storage depends on your plan:

• Business and Enterprise plans: Start with 50 GB per mailbox.
• E3 and E5 plans: Support 100 GB, plus auto-expanding archives up to 1.5 TB.

Once you cross these limits, you’ll get a notification. You’ll need to clear space or increase your quota to keep using email.

How to Check Office 365 Mailbox Size?

On Outlook Web:

1. Go to Settings (gear icon) > View all Outlook settings > General > Storage
2. You’ll see a bar showing how full your mailbox is

On Outlook Desktop:

1. Click File > Tools > Mailbox Cleanup > View Mailbox Size
2. You’ll see the total size and folder breakdown

How to Fix the “Office 365 Mailbox Is Full” Issue?

Below are 7 effective solutions you can try:

#1. Delete Unimportant Emails, Files, and Attachments

Start by cleaning your inbox:

1. Sort emails by size or date.
2. Delete old or unwanted emails.
3. Empty the Deleted Items and Junk Email folders.
4. Save large attachments to OneDrive or your local system and delete them from email.

Pro Tip: Before deleting, make a backup of important emails using a tool.

#2. Use Outlook’s Built-in Mailbox Cleanup Tool

The Mailbox Cleanup tool helps you manage mailbox size easily:

1. Go to File > Tools > Mailbox Cleanup
2. Find emails older than X days or larger than a specific size
3. Archive or delete them in bulk.

#3. Archive Emails

Enable email archiving instead of deleting:

1. In-place archive gives users an additional 50 GB to 100 GB mailbox.
2. Auto-expanding archive (available with E3/E5 plans) allows up to 1.5 TB of storage after the archive reaches 100 GB.

Note: This keeps older emails safely stored while freeing up space in your main mailbox.

#5. Migrate Emails to Another Office 365 Account

If you have another account with free space, move emails from one account to another in Office 365.

1. Use Outlook export/import or a third-party email migration tool.
2. This helps you distribute storage and maintain data safety.

#6. Upgrade Your Microsoft 365 Subscription

You can increase your mailbox storage by upgrading to a higher-tier plan:

For example, Microsoft 365 Business Standard offers more storage than the Basic plan.

E3 and E5 plans include large mailbox limits with archiving and auto-expansion.

Note: Only Microsoft 365 subscribers are eligible for additional storage features.

#7. Modify Quota Limits (Admin Only)

Admins can edit quota thresholds for warning, send, and send/receive limits:

• Warning quota: 98%
• Send quota: 99%
• Send/receive quota: 100%

This can help delay the full mailbox warning temporarily, but not beyond the maximum quota limit set by the plan.

Final Words

When your Office 365 mailbox is full, you don’t have to panic. Start by:

• Deleting unnecessary emails
• Archiving what you want to keep
• Backing up important data

If you regularly hit storage limits, consider upgrading your plan or moving old emails elsewhere.

Managing storage proactively helps you avoid disruptions and keeps your inbox running smoothly.