Fixing Missing Folders After Migration

You finished a migration, the user logged into the new mailbox, and several folders are simply not there. Often the missing folders are the ones with non-English names, customer initials, or year-based archives (2023-Q4, Clients/Müller, Архив). Sometimes they exist on the server but the mail client refuses to show them. In nearly every case the messages still exist on the destination, but they are either in a folder with the wrong name or sitting under a parent the client has hidden. This post walks through diagnosing and repairing each scenario.

WARN: Failed to CREATE folder &BCEEMARBBDcEMA-
ERROR: Folder name conflicts with existing \Noselect mailbox
INFO: Folder hierarchy depth limit reached (10)

Why this happens#

Three root causes are responsible for almost every missing-folder report.

UTF-7 encoding not decoded. IMAP uses a modified UTF-7 (defined by RFC 3501) for any folder name containing non-ASCII characters. A folder called Archiv 2024 survives intact, but a folder called Archív 2024 is sent as Arch&AO0-v 2024 on the wire. A migration tool that does not decode this writes the literal &AO0- to the destination, where the user sees a garbled name or no folder at all because the client filters it out.

Hidden \Noselect parent folders. IMAP supports folders that exist purely to hold children, marked with the \Noselect flag. Many mail clients hide these by default. If a migration creates Clients/Acme/Invoices but flags Clients and Clients/Acme as \Noselect, Outlook and Apple Mail will not show the leaf folder either, because their tree-view collapses the entire branch. The folder is on the server, the user cannot see it.

Naming conflicts on the destination. Office 365 and Gmail both reserve a number of folder names (Inbox, Sent, Drafts, Trash, Junk, Archive, and Gmail's [Gmail]/All Mail family). If the source mailbox has a custom folder with one of those exact names, the destination either refuses to create it (the migration logs an error and moves on) or merges its contents into the reserved folder, which means the original folder name disappears.

Fix it now#

1. Compare source and destination folder lists

   Run an IMAP LIST "" "*" on both source and destination, or export the folder tree from the migration tool. Diff the two lists side by side. Group the missing folders by pattern: ones with accented characters in the name, ones nested deeper than 5 levels, and ones whose name matches a reserved word on the destination. The pattern tells you which fix to apply.

2. Check for UTF-7 encoded names

   On the destination, look for folder names that begin with & or contain &...- sequences. That is undecoded UTF-7. A folder called &BCAEMARBBDcEMA- should be Архив. Mailbox Taxi flags these as UTF-7 candidates automatically; in other tools you can use imapsync --regextrans2 to decode them. The full mechanics are in the folder UTF-7 encoding fix.

3. Reveal \Noselect folders in the client

   Switch the mail client into "show all folders" mode. In Outlook this is "Show All Folders" in the navigation pane settings; in Apple Mail it is the "Get Account Info" mailbox list; in Thunderbird it is "Subscribe" on the account. If the missing folder appears once you unhide \Noselect parents, the migration succeeded but the client was filtering. Subscribe to the leaf folder and the user will see it.

4. Resolve naming conflicts

   For folders missing because their name clashes with a reserved word, rename either the source-side folder before re-running, or rename the destination-side reserved folder with an IMAP RENAME command. Office 365 will let you rename a custom Archive folder but not the system Inbox. Adding a suffix (Archive-2024) is the simplest fix and avoids any accidental merging.

5. Re-run the migration for affected folders only

   Almost every migration tool supports a folder filter. Specify only the missing folders and run again. This is much faster than redoing the whole mailbox and avoids any chance of creating duplicate emails in the folders that already completed cleanly. In Mailbox Taxi the folder filter is a comma-separated list in the job settings.

6. Validate counts after the fix

   Once the missing folders are present, compare source and destination message counts folder by folder. Follow the post-migration validation playbook. Allow a small variance for system folders (Spam, Trash) that the destination filters differently, but custom folders should match source counts exactly.

How to prevent it next time#

Pick a migration tool that decodes UTF-7 by default and surfaces it in the folder tree before the run starts, not as a log warning halfway through. If you can see the decoded names in a dry-run preview, you can spot encoding problems before any data moves.

Pre-scan the source for reserved-word collisions. Office 365 and Gmail have a known list of reserved folder names; any source folder that matches one should be renamed in the migration tool's mapping, not on the source itself, to avoid disrupting the user during their final week on the old system.

Limit nesting depth on the source where possible. IMAP allows folder hierarchies many levels deep, but most clients become difficult to use beyond 5 or 6 levels and some destinations have hard limits at 10. Flatten the tree before migrating if it is unusually deep.

Test the migration on one user who has accented characters, Cyrillic, or CJK in folder names before running the full batch. If a Müller folder survives intact, so will most of the others. The IMAP protocol glossary covers the encoding rules in more depth.