Mozilla documents four reasons why a Thunderbird folder shows no messages after a compact. Interrupted compacts produce an empty index file and Repair folder rebuilds it. Antivirus software can delete the mail file and corrupt MBOX files produce truncated files. Thunderbird 115 and older add a fourth issue with large temporary files. Mozilla does not describe any header edit.
You open Thunderbird after a crash and a folder looks empty. Your mail can still sit in the folder file. Mozilla documents what happens when a compact fails.
Facts follow the Mozilla Support article Compacting folders and potential complications. Mozilla lists Thunderbird 140 ESR through 158 and last updated it on May 26, 2024.
What does a Compact do?
Compacting rebuilds the index of a folder and saves a new MBOX mail file. Thunderbird cannot use the index while the compact is in progress. Mail of that folder remains hidden until the compact finishes.
What are the Four Complications?
- Interrupted compact. Interruption produces an empty index file with the .msf extension. No messages show. Mozilla lists backup scans, cloud sync and antivirus scans as causes because they interrupt access to the profile. Mozilla advises against cloud sync and antivirus scanning of the profile.
- Deleted mail file. Mail from before a certain date can be missing when antivirus software deleted or quarantined the mail file. Compacting exposes this because it rebuilds the index.
- Disk space. Thunderbird 115 and older can create large nstmp files in the profile directory. You can delete them.
- Corruption. Corrupt MBOX files make the compact produce a truncated file with the early messages and without the later ones. Hard disk failure, memory failure and malware are the common causes.
Other symptoms of corruption are reappearing deleted messages, messages dated 1969, mixed up message bodies, mismatched lists and content and freezing.
How to Fix an Empty Index With Repair Folder?
- Right-click the folder in Thunderbird.
- Select Properties.
- Select Repair folder.
Repair folder rebuilds the index. Mozilla states that it should recover without data loss.
What are the Warnings?
Mozilla warns that compacting can make messages appear to vanish. Some cases are recoverable and others involve permanent data loss. Mozilla advises you to post to Thunderbird support when problems persist.
What does Mozilla Not Say?
- Any step for editing the status header in the Inbox file.
- How deleted messages are stored before a compact.
- Timing of the final removal of deleted messages.
- Steps for Thunderbird versions older than the listed range.
Related reading: how to reindex Thunderbird folders and how to backup Thunderbird local folders.
Frequently Asked Questions
Why does Thunderbird Show an Empty Folder?
Mozilla lists an interrupted compact as one cause. It leaves an empty index file so no messages show.
How do I Fix an Empty Folder in Thunderbird?
Right-click the folder, select Properties and then Repair folder. Mozilla states that it rebuilds the index and should recover without data loss.
Can Antivirus Software Remove My Mail?
Yes. Mozilla states that antivirus software can delete or quarantine the mail file. Compacting then exposes the missing mail.
What Happens When the MBOX File is Corrupt?
Compacting produces a truncated file. Early messages remain and later messages are lost.
Does Mozilla Describe a Header Edit Fix?
No. Mozilla does not mention it.
Key Takeaways
- Compacting rebuilds the folder index and saves a new MBOX file.
- Interrupted compacts produce an empty index and Repair folder rebuilds it.
- Antivirus and cloud sync can interrupt a compact or delete the mail file.
- Corrupt MBOX files produce truncated files with later messages lost.
- Mozilla gives no header edit step for the Inbox file.
Sources and References
Facts follow Mozilla Support.
- Compacting folders and potential complications (Mozilla Support) support.mozilla.org