Hi!
back when we hit this issue, I did some more digging and I think I found
the root cause of the problem:
TLDR: After renaming a mailbox from non-NFC to NFC-form, the
subscription is not updated from non-NFC to NFC. This constantly
triggers a fallback code path which causes interesting side-effects.
The details:
In the beginning, the mdbox/subscriptions file contains an entry in
non-NFC mUTF7: 'DA&Awg- KNATSCH' The directory in mdbox/mailboxes also
has this non-NFC mUTF7 name: 'DA&Awg- KNATSCH'.
As soon as an IMAP client lists the folders, Dovecot silently renames
the non-NFC mailbox directory to its NFC form on disk, as seen in the
logs with 'mail_debug = yes':
Debug: Mailbox 'DÄ KNATSCH' renamed for NFC normalization
In mdbox/mailboxes, the directory is now called 'D&AMQ- KNATSCH'. But
the entry in mdbox/subscriptions is not updated and still contains the
non-NFC form 'DA&Awg- KNATSCH'.
The next time a client connects, it again tries to access the 'DA&Awg-
KNATSCH' folder.
Now Dovecot tries _again_ to rename 'DA&Awg- KNATSCH' into its NFC-form.
But this fails, because the target directory 'D&AMQ- KNATSCH' already
exists.
This in turn triggers the fallback code path which now tries to rename
'DA&Awg- KNATSCH' into something like 'D&AMQ-
KNATSCH-584c5a01303f196a26742c00911dccb7'. This also fails because the
non-NFC source directory 'DA&Awg- KNATSCH' doesn't exist anymore. This
error triggers the message we see in the logs:
Error: Failed to rename mailbox 'DÄ KNATSCH' for NFC normalization:
Mailbox doesn't exist: DÄ KNATSCH
At this point, the folder shows up as unsubscribed in Roundcube. If I
resubscribe in Roundcube, I can access the mails fine. The
mdbox/subscriptions file now has two entries:
DA&Awg- KNATSCH
D&AMQ- KNATSCH
So the root problem seems to be the non-NFC-entry in mdbox/subscriptions
which doesn't get updated after renaming the mailbox to its NFC form.
# List the mailboxes of the user to trigger a rename of the directory
$> bin/doveadm mailbox list -u non-nfc-user
...
DÄ KNATSCH
...
# Resubscribe with the output from the above command. This adds the
correct subscription.
$> bin/doveadm mailbox subscribe -u non-nfc-user "DÄ KNATSCH"
I tried to unsubscribe the non-NFC form, but that only unsubscribes the
NFC form. Apparently the non-NFC form gets normalized to NFC before
unsubscribing.
But wait, there's more!
As long as the non-NFC entry stays in mdbox/subscriptions, we hit the
fallback codepath whenever something iterates over the mailboxes.
Because of this, Dovecot does not report the non-NFC mailbox name in its
subscription list, but the one with the GUID-suffix:
$> doveadm mailbox list -s -u non-nfc-user
doveadm(non-nfc-user): Error: Failed to rename mailbox 'DÄ KNATSCH' for
NFC normalization: Mailbox doesn't exist: DÄ KNATSCH
DÄ KNATSCH <-- This is the NFC subscription
...
DÄ KNATSCH-a0fdd806a250196a6aaa2c00911dccb7 <-- the broken non-NFC
subscription
Roundcube just seems to ignore the GUID one because no such mailbox exists.
So I think updating the mailbox name to NFC should also update any
matching subscriptions of the user. Otherwise, the subscription gets
lost and we get constant errors in the logs.
HTH,
Michael
On 7/3/26 15:55, Jens Urban via dovecot wrote:
Hi,
During the upgrade from Dovecot 2.3.21.1 (d492236fa0) to 2.4.4-5+debian12
(8b687aa65c), we noticed that this error also occurred in our environment.
The error appears repeatedly in the log file. Unfortunately, we were not
able to reproduce it manually. We tried many operations with the affected
mailbox using our webmail frontend, but none of them triggered the error.
More or less by accident, we found a simple way to reproduce it:
"doveadm mailbox list -u USER -s"
Without the "-s" option, the error does not occur.
The attached information may be useful for debugging. It seems helpful
because the problem can now be reproduced on demand. Maybe the "-s" code
path (subscribed mailboxes) gives a hint where the problem comes from.
Please see the attached observations and hex dumps for more details (the
strace mailbox name comes from "strace doveadm mailbox list -u USER -s)"
:
Regards,
Jens
----------------------------------------------
Name from strace:
Fu&Awg-hrerschein
00000000 46 75 26 41 77 67 2d 68 72 65 72 73 63 68 65 69
|Fu&Awg-hrerschei|
00000010 6e 0a |n.|
00000012
Name from filesystem (ls -Nd):
F&APw-hrerschein
00000000 46 26 41 50 77 2d 68 72 65 72 73 63 68 65 69 6e
|F&APw-hrerschein|
00000010 0a |.|
00000011
doveadm mailbox list -7
INBOX.F&APw-hrerschein
00000000 49 4e 42 4f 58 2e 46 26 41 50 77 2d 68 72 65 72
|INBOX.F&APw-hrer|
00000010 73 63 68 65 69 6e 0a |schein.|
00000017
doveadm mailbox list -7 -s
doveadm(xx@xx): Error: Failed to rename mailbox 'INBOX.Fuehrerschein' for
NFC normalization: Mailbox doesn't exist: INBOX.Fuehrerschein
INBOX.F&APw-hrerschein-6960360bc4b6476ae3362000ca608b0a
doveadm(xx@xx): Error: Failed to rename mailbox 'INBOX.Fuehrerschein' for
NFC normalization: Mailbox doesn't exist: INBOX.Fuehrerschein
00000000 49 4e 42 4f 58 2e 46 26 41 50 77 2d 68 72 65 72
|INBOX.F&APw-hrer|
00000010 73 63 68 65 69 6e 2d 36 39 66 32 35 33 31 30 63
|schein-69f25310c|
00000020 34 62 36 34 37 36 61 66 31 33 36 32 30 30 30 63
|4b6476af1362000c|
00000030 61 36 30 38 62 30 61 0a |a608b0a.|
00000038
doveadm mailbox list -8
INBOX.Fuehrerschein
00000000 49 4e 42 4f 58 2e 46 c3 bc 68 72 65 72 73 63 68
|INBOX.F..hrersch|
00000010 65 69 6e 0a |ein.|
00000014
doveadm mailbox list -8 -s
doveadm(xx@xx): Error: Failed to rename mailbox 'INBOX.Fuehrerschein' for
NFC normalization: Mailbox doesn't exist: INBOX.Fuehrerschein
doveadm(xx@xx): Error: Failed to rename mailbox 'INBOX.Fuehrerschein' for
NFC normalization: Mailbox doesn't exist: INBOX.Fuehrerschein
00000000 49 4e 42 4f 58 2e 46 c3 bc 68 72 65 72 73 63 68
|INBOX.F..hrersch|
00000010 65 69 6e 2d 38 31 62 35 61 30 32 34 63 34 62 36
|ein-81b5a024c4b6|
00000020 34 37 36 61 31 62 33 37 32 30 30 30 63 61 36 30
|476a1b372000ca60|
00000030 38 62 30 61 0a |8b0a.|
00000035
Von: Michael Goth via dovecot <[email protected]>
Datum: Mittwoch, 20. Mai 2026 um 09:07
An: [email protected] <[email protected]>
Betreff: Re: NFC normalization rename errors after 2.3 to 2.4 upgrade
(mdbox)
Hi everyone,
we also hit this bug(?) last week. Like Jan, we were upgrading from 2.3
to 2.4.3. 'mailbox_list_index_very_dirty_syncs' is 'no' in our setup.
After the update, folders with a 32-hex GUID suffix started to appear.
The only thing that stopped it from spreading, was the setting
'mailbox_list_normalize_names_to_nfc = no'.
I cleaned up the folders with a script, using the method Jan described.
After Philip's encouraging comment, I upgraded one of our servers to
2.4.4 and removed 'mailbox_list_normalize_names_to_nfc = no' from its
config. But the same error started to appear in the logs again:
Error: Failed to rename mailbox 'Infos, Verta*ge, etc' for NFC
normalization: Mailbox doesn't exist: Infos, Verta*ge, etc
But there's a difference: No folder with a 32-hex GUID was suffix
created. In the example from the log message above, there's still only
one folder on disk, even after the error appeared:
mdbox/mailboxes/Infos, Vert&AOQ-ge, etc
The mails in the folder are still accessible, but it no longer appears
as subscribed in Roundcube Mail.
But for other folders, we again got the GUID-suffixed "copy", like in
Dovecot 2.4.3.
We switched back to 'mailbox_list_normalize_names_to_nfc = no' for now.
So far, I failed to reproduce the issue on a different server with
Dovecot 2.4.4. On this server, I can create (nested) folders with
umlauts, put mails into them, etc. - The error never appears in the
logs, everything works fine.
I'm not sure what to do next.
Regards,
Michael
On 5/17/26 23:34, Philip Iezzi via dovecot wrote:
> Hi Timo
> No, I don't have mailbox_list_index_very_dirty_syncs=yes, using
default no
> value.
> I can no longer reproduce this problem since having upgraded from
2.4.3 to
> 2.4.4, no single "Error: Failed to rename mailbox '...' for NFC
> normalization" error since then.
> Previously, on Dovecot 2.4.3, the Errors also popped up on regular
> umlauts, even after renaming those characters to regular umlauts to
make
> sure those were not represented as NFD. But on Dovecot 2.4.4 all
good,
> without any further workarounds needed.
> May this be related to this changelog entry?
> - lib-storage: Auto-rename non-NFC subscription file entries to NFC
on
> read.
> Kind regards,
> Philip
>
> On 16 May 2026, at 13:53, Timo Sirainen via dovecot
> <[email protected]> wrote:
> Do you have mailbox_list_index_very_dirty_syncs=yes? If yes, does
the
> problem go away if you run: doveadm -o
> mailbox_list_index_very_dirty_syncs=no mailbox list -u $user
>
> Alternatively, try doveadm force-resync -u $user '*'
>
> Yet another possibility to try rm dovecot.list.index*
>
> Also, there is this patch that possibly helps:
>
> diff --git a/src/lib-storage/mail-storage.c
> b/src/lib-storage/mail-storage.c
> index a4363fc73c..b42053bc1a 100644
> --- a/src/lib-storage/mail-storage.c
> +++ b/src/lib-storage/mail-storage.c
> @@ -1557,11 +1557,7 @@ int mailbox_rename_nfc_forced(struct
mailbox_list
> *list, const char *vname_raw,
>
> ret = mailbox_rename(box_old, box_new);
> if (ret < 0 &&
> - mailbox_get_last_mail_error(box_old) ==
MAIL_ERROR_NOTFOUND)
> {
> - /* ignore */
> - ret = 0;
> - } else if (ret < 0 &&
> - mailbox_get_last_mail_error(box_old) ==
> MAIL_ERROR_EXISTS) {
> + mailbox_get_last_mail_error(box_old) ==
MAIL_ERROR_EXISTS) {
> /* generate a new unique name */
> guid_128_t guid;
> guid_128_generate(guid);
> @@ -1572,6 +1568,16 @@ int mailbox_rename_nfc_forced(struct
mailbox_list
> *list, const char *vname_raw,
> box_new = mailbox_alloc(list, *vname_new_r, 0);
> ret = mailbox_rename(box_old, box_new);
> }
> + if (ret < 0 &&
> + mailbox_get_last_mail_error(box_old) ==
MAIL_ERROR_NOTFOUND)
> {
> + /* Source doesn't exist on disk anymore (e.g.
backing
> store
> + already renamed externally while the list index
still
> has
> + the old non-NFC entry). Treat as no-op rather
than
> fail
> + the iteration: return the plain NFC name so the
> caller
> + doesn't expose the unique-suffix fallback name.
*/
> + *vname_new_r = vname_nfc;
> + ret = 0;
> + }
> if (ret < 0)
> *error_r = mailbox_get_last_error(box_old, NULL);
> mailbox_free(&box_old);
>
> On 12. May 2026, at 15.43, Philip Iezzi via dovecot
> <[email protected]> wrote:
>
> Thanks Jan for reporting this. I also experience this Bug since
having
> upgraded Dovecot 2.4.2 -> 2.4.3 on 2026-03-27, leading to such
errors
> in mail.log:
>
> Error: Failed to rename mailbox 'INBOX/Zuerich' for NFC
normalization:
> Mailbox doesn't exist: INBOX/Zuerich
>
> - Zuerich (in above log line) uses a decomposed sequence: a
plain u
> (U+0075) followed by a combining diaeresis (U+0308), which the
> renderer stacks on top.
> - Zuerich (which we usually type) uses a precomposed character:
ue is
> a single code point, U+00FC (LATIN SMALL LETTER U WITH
DIAERESIS).
>
> Such errors lead to broken folder structure - the user is no
longer
> able to list the whole folder structure (except INBOX) and on
mail
> clients like Roundcube, all folders appear as no longer
subscribed.
> Renaming the "ue" to "ue" fixes the problem. But I have a bunch
of
> other customers which use such characters and in previous
Dovecot
> versions <= 2.4.2 this was never a problem.
>
> Is there any known issue / will this be addressed in the next
minor
> version?
>
> Kind regards,
> Philip
>
> On 5 May 2026, at 16:04, Jan Muennich via dovecot
> <[email protected]> wrote:
>
> Following up on my own thread in case anyone hits the same
issue:
>
> The failed NFC rename in 2.4 left behind a duplicate mailbox
with a
> 32-hex GUID suffix, but only for mailboxes that had
subfolders. So
> for every affected name we ended up with two entries:
>
> Antraege
> Antraege-e078d427706eda694b210000a386c91b
>
> The original kept its (broken) name with no message access,
and the
> GUID-suffixed sibling held the actual mail. Both showed up as
real
> folders in IMAP clients.
>
> I figured out from the source code that the failed rename hits
a
> fallback path that creates the GUID-suffixed sibling, but not
what
> triggers the failure in the first place. I also found a hidden
> option there that is not listed in documentation, which works
as a
> workaround to stop new occurrences:
> 'mailbox_list_normalize_names_to_nfc = no'.
>
> Fix for already-affected mailboxes: for each one, create a
temporary
> ASCII-named replacement at the same hierarchy level, move the
> subfolders into it, move messages from the GUID-suffixed
mailbox
> into it, delete the GUID-suffixed mailbox (the broken original
then
> disappears), and rename the replacement back to the original
name.
> The final rename runs through the regular code path (without
NFC
> normalisation) and works fine.
>
> Kind regards,
> Jan
>
> On 11 Apr 2026, at 17:25, Jan Muennich via dovecot
> <[email protected]> wrote:
>
> Hi,
>
> We just upgraded from 2.3 to 2.4.3 and are now getting
errors like
> this for any mailbox with German umlauts in the name:
>
> "Error: Failed to rename mailbox 'Antraege' for NFC
normalization:
> Mailbox doesn't exist: Antraege"
>
> The mailboxes work fine otherwise, doveadm mailbox status
shows
> them with correct message counts. The errors show up on IMAP
> access but don't seem to break anything.
>
> We're using mdbox. On disk the directory is stored as
mUTF-7:
>
> Antr&AOQ-ge
>
> doveadm mailbox list gives back the name with c3 a4 for the
ae,
> which is already NFC. We double-checked with python3
unicodedata
> and it confirms the name is NFC. So it seems like dovecot is
> trying to rename something that doesn't need renaming, and
then
> failing.
>
> Is there a way to turn off this automatic rename? Or is this
a
> known issue with mdbox and mUTF-7 directory names?
>
> Any help would be appreciated to find out if this is a bug
or a
> configuration issue on our side!
>
> Thanks,
> Jan
>
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Michael Goth
.webflow GmbH
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--
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.webflow GmbH
Geschäftsführer: Andreas Schrei
Wasserburger Straße 4
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Amtsgericht Traunstein HRB 18537
E-Mail: [email protected]
Tel: +49 (0) 8621 - 99989 - 26
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Web: www.webflow.de
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