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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. I don't think any existing software
    can cause this.
 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> [MailLocation.txt]
       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 me know how. Dovecot versions earlier than 1.0.rc27 have some known bugs.

UIDVALIDITY changes
-------------------

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.

Crashes
-------

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
[http://dovecot.org/tools/mbox-anonymize.pl] and send it, the mailbox's
'dovecot.index' and 'dovecot.index.log' files to tss@iki.fi. 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>
[LDA.txt] 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')

(This file was created from the wiki on 2019-06-19 12:42)