Troubleshoot Thunderbird Mailbox Size Limit for Mac & Windows

  

Published By Karen Chard 

Approved By Rollins Duke

Published On February 27th, 2026

Reading Time 6 Min Read

So if you are having problems sending and receiving emails in Thunderbird, performance issues, MBOX corruption, or errors such as “UID max mailbox size pops up”. It is likely because the Mozilla Thunderbird email size limit is increasing.

This complete guide elaborates on how to identify when the Thunderbird storage limit exceeds, how to fix the mailbox size, and what the preventive measures are to avoid the error.

How to Check if Thunderbird Mailbox Size is Getting Too Large?

Generally, users get issues with performance when the Thunderbird mailbox size limit is exceeded.

Check your mailbox/email folder size:

1. Open Thunderbird.
2. Right-click the Mailbox folder you want to know the size of.
3. Select Properties.
4. View Size on Disk.

Find the Total Thunderbird Mailbox Size Limit (Profile Size):

If the file size is about to reach the maximum disk space of your Mac or Windows desktop. You should consider cleaning up Thunderbird mailboxes.

1. Go to the Menu (three lines) in Thunderbird.
2. Select Help, then Troubleshooting Information.
3. Go to the Profile Folder, then click Open Folder.
4. Press Alt + Up Arrow.
5. Right-click the default release folder and choose Properties.
6. View Size or Size on Disk.

Fix Thunderbird Exceeded Mailbox Size Limit Issues

Mozilla Thunderbird offers many built-in ways to manage storage limits/space. If you are nearing the maximum capacity or your Thunderbird is getting slow because mailboxes are getting too large. There are a few solutions you can try.

Solution 1: Delete Unnecessary Emails for Thunderbird Account

Too many emails in an abandoned account can unnecessarily make the Thunderbird mailbox too large.

Right-click and delete unessential mailboxes like Spam, old business Newsletters, community & promotional emails to avoid the Quota Exceeded error.

Empty out the Trash folder to reach Thunderbird mailbox size limit.

To recover disk space, Thunderbird offers to delete old messages permanently, both local copies and originals on the remote server, in one go.

1. Open Thunderbird email application.
2. Click on the Settings icon and select Accounts settings.
3. Go to the Synchronization & Storage page.
4. Go to Disk Space.
5. Enable Delete All but Most Recent.
6. Set the limit to 2000.

Solution 2: Compact Thunderbird Folders

After deleting unnecessary emails, Thunderbird email size limit may still be exceeded. This is because deleting them and removing them from the Trash/Deleted folder does not remove them from the hard drive storage in the Profile.

Comparing folders helps you avoid Thunderbird’s mailbox size limit by removing them from the profile.

1. Select all the large mailboxes in the Thunderbird account.
2. Right-click and select Compact.
3. You can see the saved storage space in the actions tab at the bottom.

Solution 3: Change Thunderbird Message Store Type to Avoid Storage Limit

Mozilla Thunderbird can save emails locally in 2 formats – MBOX & Maildir. Maildir offers better organization as compared to MBOX, thus helping resolve the storage limit error.

1. When configuring the account.
2. Click Edit Configuration, then Advanced Settings.
3. Select Server Settings from the left panel.
4. Scroll to Message Store Type under Message Storage.
5. Choose Maildir instead of MBOX.

Solution 4: Restrict Local Storage of IMAP Email in Thunderbird

Although Thunderbird does not have a Thunderbird mailbox size limit, it allows you to put a cap on the disk storage quota of IMAP account emails stored locally.

To save disk space, downloading messages from the server for offline use can be restricted by age or size.

1. Open the Navigation Menu and go to Account Settings.
2. Go to Synchronisation & Storage in the left panel.
3. Scroll to the Disk Space Section at the bottom.
4. Enable Synchronise the most recent option.
5. Set the limit to 1year.

Solution 5: Set Up a New Profile if Thunderbird Mailbox Size Limit Increases

Setting up a new profile with the Maildir saving option is the best way to avoid corruption if the Thunderbird mailbox is getting too large. It keeps a copy of the original profile, and no email is lost.

1. Close Thunderbird email client.
2. Press Win+R in Windows.
3. Type thunderbird.exe -P.
4. In Profile Manager, select Create Profile.
5. Name your New Profile and press Finish.
6. Select your New Profile. Check Use selected profile without asking at startup box and Start Thunderbird.