“Cached Message Size Smaller Than Expected” Error in Dovecot

Dovecot is a widely used IMAP and POP3 server for Linux-based mail systems. However, administrators sometimes encounter errors related to message size discrepancies in the mail cache. One such error is:

Error: Mailbox INBOX: UID=XXXXX: read(/path/to/maildir, S=2316, W=2364:2,) failed: Cached message size smaller than expected (2316 < 5914, box=INBOX, UID=XXXXX)

This error indicates a mismatch between the expected message size and the cached size, leading to potential issues when accessing emails. In this article, we will explore the causes and solutions for this error.

Causes of the Error

Several factors can contribute to this issue:

1. Corrupted Dovecot Index or Cache Files: Dovecot maintains index and cache files to improve performance. Corruption in these files can lead to inconsistencies.
2. Incomplete or Truncated Email Files: If an email message was not fully written to disk due to a system crash, disk space issues, or abrupt termination, its size may not match the cached size.
3. File System Issues: Disk errors or file system corruption can result in missing or incomplete message files.
4. Interrupted Synchronization: If mail synchronization was interrupted (e.g., during a migration or backup process), cache files may become outdated.

Steps to Fix the Issue

Follow these steps to resolve the error:

1. Check Dovecot Logs for Clues

Use the following command to check for additional errors in the logs:

journalctl -u dovecot --since "1 hour ago"

Alternatively, check the system log:

tail -f /var/log/mail.log

2. Inspect the Corrupt Message

Identify the problematic email file using:

ls -lh /home/vmail/example.com/user/Maildir/cur/

Attempt to view its content:

cat /home/vmail/example.com/user/Maildir/cur/<filename>

If the message appears truncated or corrupted, consider removing it.

3. Rebuild Dovecot Indexes

Try reindexing the mailbox:

doveadm index -u user@example.com INBOX

If the issue persists, manually delete the index and cache files:

rm -rf /home/vmail/example.com/user/Maildir/dovecot.index*
rm -rf /home/vmail/example.com/user/Maildir/dovecot.cache*

Then restart Dovecot:

systemctl restart dovecot

4. Check for Disk or File System Errors

Run the following commands to check for disk space and file system issues:

df -h  # Check available disk space
dmesg | grep -i error  # Look for disk-related errors
fsck /dev/sdX  # Replace sdX with the correct partition

5. Remove the Corrupt Email (If Necessary)

If the issue persists and a specific email is causing it, move the file to a temporary location:

mv /home/vmail/example.com/user/Maildir/cur/<filename> /home/vmail/example.com/user/Maildir/tmp/

This ensures that Dovecot does not attempt to process the corrupted email while preserving it for investigation.

Conclusion

The “Cached message size smaller than expected” error in Dovecot typically arises from index corruption, file system inconsistencies, or interrupted mail delivery. By checking logs, inspecting message files, rebuilding indexes, and verifying disk health, administrators can efficiently resolve the issue and restore normal mail operations.

If you continue to experience issues, consider enabling verbose logging in Dovecot for further troubleshooting.

For more technical articles, stay tuned to our blog!