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:

Anträge Anträge-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 Münnich via dovecot <dovecot@dovecot.org> 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 'Anträge' for NFC normalization: Mailbox doesn't exist: Anträge"

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 ä, 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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