mbox problems

External modifications

In general Dovecot doesn’t mind if you modify the mbox file externally. It’s fine if external software expunges messages or appends new ones. However moving around existing messages, inserting messages in the middle of the file or modifying existing messages isn’t allowed.

Especially modifying existing messages (eg. removing attachments) may cause all kinds of problems. If you do that, at the minimum go and delete dovecot.index.cache file from the mailbox, otherwise weird things may happen. However IMAP protocol guarantees that messages don’t change at all, and deleting Dovecot’s cache file doesn’t clear clients’ local caches, so it still may not work right.

If you insert messages, or if you “undelete” messages (eg. replace mbox from a backup), you may see errors in Dovecot’s logs:

mbox sync: UID inserted in the middle of mailbox /home/tss/mail/inbox (817 > 787, seq=18, idx_msgs=32)

This is normal. Dovecot just assigned new UIDs for the messages. See below for other reasons why UID insertions could happen.

Debugging UID insertions

The above error message can be read as: “18th message in the mbox file contained X-UID: 787 header, however the index file at that position told the message was supposed to have UID 817. There are 32 messages currently in the index file.”

There are four possibilities why the error message could happen:

  1. Message with a X-UID: 787 header really was inserted in the mbox file. For example you replaced mbox from a backup.

  2. Something changed the X-UID headers. Very unlikely.

  3. The message was expunged from the index file, but for some reason it wasn’t expunged from the mbox file. The index file is updated only after a successful mbox file modification, so this shouldn’t really happen either.

  4. If this problem happens constantly, it could mean that you’re sharing the same index file for multiple different mboxes!

    • This could happen if you let Dovecot do mailbox autodetection and it sometimes uses /var/mail/%u (when it exists) and other times ~/mail/inbox. Use an explicit mail_location setting to make sure the same INBOX is used.

    • Another possibility is that you’re sharing index files between multiple users. Each user must have their own home directory.

It’s possible that broken X-UID headers in mails and mbox_lazy_writes=yes combination has some bugs. If you’re able to reproduce such an error, please let us know how.


UIDVALIDITY is stored in X-IMAPbase: or X-IMAP: header of the first message in mbox file. This is done by both Dovecot and UW-IMAP (and Pine). It’s also stored in dovecot.index file. It shouldn’t normally change, because if it does it means that client has to download all the messages for the mailbox again.

If the UIDVALIDITY in mbox file doesn’t match the one in dovecot.index file, Dovecot logs an error:

UIDVALIDITY changed (1100532544 -> 1178155834) in mbox file /home/user/mail/mailbox

This can happen when the following happens:

  1. Dovecot accesses the mailbox saving the current UIDVALIDITY to dovecot.index file.

  2. The UIDVALIDITY gets lost from the mbox file

    • X-IMAP: or X-IMAPbase: header gets lost because something else than Dovecot or UW-IMAP deletes the first message

    • The whole file gets truncated

    • Something else than Dovecot deletes or renames the mbox

  3. The mailbox is accessed (or created if necessary) by UW-IMAP or Pine. It notices that the mailbox is missing UIDVALIDITY, so it assigns a new UIDVALIDITY and writes the X-IMAPbase: or X-IMAP: header.

    • Also Dovecot that’s configured to not use index files behaves the same.

  4. Dovecot accesses again the mailbox. UIDVALIDITY in the mbox file’s header doesn’t match the one in dovecot.index file. It logs an error and updates the UIDVALIDITY in the index file to the new one.


Dovecot’s mbox code is a bit fragile because of the way it works. However instead of just corrupting the mbox file, it usually assert-crashes whenever it notices an inconsistency. You may see crashes such as:

Panic: mbox /home/user/mail/mailbox: seq=2 uid=45 uid_broken=0 originally needed 12 bytes, now needs 27 bytes

This is a bit difficult problem to fix. Usually this crash has been related to Dovecot rewriting some headers that were broken. If you see these crashes, it would really help if you were able to reproduce the crash.

If you have such a mailbox which crashes every time when it’s tried to be opened, please put the mbox through mbox anonymizer and send it, the mailbox’s dovecot.index and dovecot.index.log files to dovecot@dovecot.org (see https://dovecot.org/mailman3/mailman3/lists/dovecot.dovecot.org/. None of those files contain any actual message contents so it’s be safe to send them.

Avoiding crashes and errors

Since the problems usually have been related to broken headers, you should be able to avoid them by filtering out all the Dovecot’s internal metadata headers. This is a good idea to do in any case. If you use Dovecot LDA it does this filtering automatically. Otherwise you could do this in your SMTP server. The headers that you should filter out are:

  • Content-Length

  • Status

  • X-IMAP

  • X-IMAPbase

  • X-Keywords

  • X-Status

  • X-UID

  • X-UIDL (if you’re using pop3_reuse_xuidl=yes)