Multiple nested inbox folders in "Mail"

Some users are experiencing an issue in which Mail.app creates more and more inboxes that are nested within each other. Sometimes, at first, only a few nested mailboxes will be created, but then more will be added over time.

Apple Dicussions poster bionicDan writes:

"I have this strange issue that keeps popping up. Within one of my mail accounts, accessed via IMAP, a set of nested INBOX folders is created."

Several affected users have been using different Mail clients to access the affected accounts, including Mail on the iPhone and Thunderbird on Windows PCs, but it appears that Mail.app is the cause of the issue. Manually deleting the nested mailboxes on the server is only a temporary fix, because the problem reoccurs. Some suspect that the problem is an improperly configured Mail server, as several users have been running the DreamHost server software, but this happens on several other software packages as well. Additionally, users have experimented with the problem and found that, when they do not use Mail, the inboxes stay intact, but after solely using Mail for a while, the problem starts in again. This has started for some users after upgrading the system to Mac OS X 10.5.5, but has occurred for others in other versions of the operating system as well.

In IMAP accounts, users have the option to specify their root folder for Mail accounts (also known as the "IMAP Path Prefix"), which isolates folders the mail program uses from others in the user's account on the email server. When users have experimented with the prefix entry some were able to fix the problem, even though this has not been the case for others.

Potential Fixes

Change the IMAP Path Prefix Go to Mail's preferences and in the "Accounts" section select the problematic account. Then go to the "Advanced" tab and change the field to the value given by the account provider. If the problem persists, users might try changing the case of the path prefix entry. For instance, with Google's Gmail, the default is to use "[Gmail]" as the prefix, but users might try "[GMAIL]" or "[gmail]" as well. In addition, since the prefix is actually a folder path, users might try putting a forward slash "/" at the end of the prefix, which may help indicate that the path ends in a folder. Some users might also have success with adding the "INBOX" name at the end of the IMAP prefix, but it is recommended that users try both with and without this.

Change Mailbox Behavior In the screenshot posted by one affected user, the Trash mailbox is at the end of the tree of nested inboxes. This could indicate the problem lies in how the Trash folder is being dealt with. As such, users might try toggling combinations of the options to "Move deleted messages to the Trash mailbox", and "Store deleted messages on the server", which are available in the "Mailbox Behaviors" tab in the "Accounts" preferences.

Reset Mail's preferences The IMAP prefix settings are stored in the account settings in the preferences files for Mail, and if there is a corruption in the preference files that is preventing settings changes from working correctly, then this could be why the problems reoccur for some users. The Mail preferences file is called "com.apple.Mail.plist" and is located in the /username/Library/Preferences/ folder. Removing this file will require users to re-enter account information, but Mailbox setup and downloaded messages will all remain intact.

NOTE: For both of these potential fixes users may have to manually delete the nested inboxes that have been created.

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