Problem connecting to INBOX.subfolder in combination with IMAP namepace prefix "INBOX."; Error [NONEXISTENT]; Assuming wrong URL.

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App

K-9 Mail

App version

19.0

Where did you get the app from?

F-Droid

Android version

LineageOS 19 (Android 12) & GrapheneOS 16

Steps to reproduce

Select new IMAP account, use auto-config. Refresh folder list. Try to access emails contained by "INBOX.subfolder"s.

IMAP Server is dovecot with standard namespace prefix "INBOX." being set. This cannot be changed as it is a default given by web-host.

Subfolders of Inbox (INBOX subfolder 1, etc.) cannot be accessed, synced, contents are not displayed. These folders seem to be addressed incorrectly.
Despite they are discovered (by "refresh folder list" command) and listed nicely nested under Inbox.

K9mail - Error Log states:
    Command: EXAMINE "INBOX.subfolder 1"; response: #11# [NO, [NONEXISTENT], Mailbox doesn't exist: INBOX.subfolder 1 (0.001 + 0.000 secs).]

Expected behavior

Access, sync, and display emails and contents of the specified sub-folders correctly.

Actual behavior

Subfolders of Inbox (INBOX subfolder) seem to be addressed incorrectly.

While other top level folders and also sub folders are displayed and synced correctly.

Also tested:
Old version of K-9 (5.600) does not yet do the nested displaying of sub-folders (as on screenshot) but instead shows these folders as INBOX.subfolder. But here all folders are correctly recognized and also synced without problems.
Also tested:
Other Mailclient.app also shows folder-sort differently but access to same folders are working.

Already tried to set IMAP prefix manually to (INBOX.) instead, with "auto-detect IMAP namespace" turned OFF.
This did not help.

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Please fix soon.
These folders are important and cannot be accessed from our mobile devices.

(My research also lead me to Post #1342; Maybe related.
This could possibly also be fixed with correct namespace addressing.)

Logs

k9mail-log_2026-05-32_Connect-Error.txt

Reproduce the issue by configuring a Dovecot account with the standard IMAP namespace prefix "INBOX." and refreshing the folder list. Compare how nested Inbox folders are discovered with the EXAMINE command shown in the error log, and use the reported Post #1342 as related context. Done means Inbox subfolders can be accessed, synced, and displayed correctly without the NONEXISTENT error.