Compacting folders

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

When you delete or move e-mails from a folder, Thunderbird and other e-mail programs simply hide them (and mark them as ready for permanent deletion), and these hidden e-mails still actually remain in the folder. Even emptying the Trash does not permanently delete them. These hidden e-mails are permanently removed only when the folder is compacted. If you don’t compact your mail folders periodically then they can grow very large and erratic program behavior may occur.

E-mail users have often never heard of compacting folders (not to be confused with compressing files). However, this approach to e-mail storage and deletion is common to most e-mail clients. The difference is that most other email clients by default automatically compact the folder when a certain amount of space is wasted; Thunderbird does not do this.

How to compact folders

To compact all folders in an account, click the account on the left, and then click "File -> Compact Folders". Compacting an account may take from a few seconds to 20 minutes or more, depending on how much mail you have and how recently you last compacted the folders. If you have trouble doing this and the process stalls, try compacting one folder at a time by right-clicking on the folder and choosing "Compact This Folder".

Try to avoid downloading e-mail while compacting folders, as it has been reported that this can cause corruption or a "folder being processed" error. For POP3 accounts, if Offline Support is installed, you should go offline before compacting folders. (To do this, go to "File -> Offline -> Work Offline", or simply click on the icon in the bottom left corner.) For IMAP accounts you cannot compact folders unless you are online.

Other ways to compact folders

For IMAP accounts, Thunderbird can automatically compact the Inbox when you exit: click "Tools -> Account Settings -> [account name] Server Settings", and check "Clean up ('Expunge') Inbox on Exit." If you check "Empty Trash on Exit" it will empty and compact the Trash folder when you exit.

If Offline Support is installed, you can have Thunderbird prompt you to compact folders when it will save a certain amount of disk space. Go to "File -> Offline -> Offline Settings", and check "Compact folders when it will save over ___ KB". Advanced users can set the following preferences to configure this feature, even without Offline Support:

user_pref("mail.prompt_purge_threshhold", true);
user_pref("mail.purge_threshhold", XXX);         //where "XXX" is the size threshold in kB

This will ask you for permission whenever it wants to compact the folder. (These two preferences only affect messages stored on your hard disk. If you have an IMAP account they will not compact your remote folders.)

You can use the AutoMsgSelect extension to automatically press the "OK" button for you, so that the dialog box just flashes on the screen and then disappears.

Problems when compacting

Compacting seems to take forever

Usually it only takes a couple of minutes to compact your folders unless your profile is over a gigabyte in size or your computer is slow. The worst that normally happens if you exit Thunderbird before it finishes compacting is that it fails to delete some nstmp temporary files that it created and you have to manually delete them.

If compacting takes a long time, the status bar at the bottom of the window may not remove the text about it compacting a folder when it finishes. It may also fail to update that text to mention the last folder in that account if you use "File -> Compact Folders". If you do something else that normally displays some text in the status bar it should replace the message about compacting.

Compacting doesn’t seem to work

If compacting folders doesn't seem to work even for a single folder (where you right-click on the folder and choose "Compact This Folder"), it is possible that the .msf mail summary files are corrupted. To fix this problem:

1. Exit Thunderbird.
2. Make a temporary backup copy of your profile folder.
3. In your profile folder, delete the files ending in .msf. Thunderbird recreates them when it starts. (If you’re using Windows and want a quick way to find all msf files in all subfolders too, just enter *.msf in Start->Search. On Windows XP, ensure that "Search hidden files" is chosen under "More advanced options"; on earlier versions of Windows, ensure that hidden files are viewable in Windows Explorer ("Tools -> Folder Options -> "View hidden files and folders").

If the problem persists and there is one particular folder (such as your Inbox) that fails to be compacted, you can try the following:

1. Exit Thunderbird and make a temporary backup copy of your profile folder so that you can revert everything easily if necessary.
2. Start Thunderbird, create a new folder and name it "TestFolder" (or similar).
3. In the folder that is giving you problems (e.g. Inbox), select all the messages and copy them to TestFolder ("Message -> Copy" -> [account name] -> TestFolder").
4. Verify that all of the messages have been copied to TestFolder.
5. Right-click on TestFolder and choose "Compact This Folder".
6. Go to your profile folder and see if TestFolder looks like it has been compacted. It should be significantly smaller in size than the folder from which you copied the messages.
7. If it looks like TestFolder can in fact be compacted successfully, exit Thunderbird, and go to your profile folder. Then:
   1. Rename the file that was giving you problems (e.g. rename "Inbox" to "InboxOLD") and delete its .msf file (e.g. "Inbox.msf").
   2. Rename the "TestFolder" folder to "Inbox" (or whatever the problematic folder was called) and delete "TestFolder.msf".
   3. Restart Thunderbird, and use it as usual. If compacting folders seems to work correctly during a test period (such as one week) then you can delete the folder "InboxOLD".

What if you don’t compact folders?

If you rarely or never compact folders, your mail files will accumulate more and more of the "hidden" messages that has been marked for permanent deletion but has not yet been removed. This can cause a lot of disk space to be used, and it can have a negative effect on Thunderbird’s performance.

• Even if a mail folder seems to be empty or nearly empty, the mail file can become very large [1][2][3]. This wastes disk space, and when you back up your mail files for safekeeping, you will waste time backing up all these "hidden" messages as well.
• When downloading messages, you might occasionally get duplicates of messages you’ve already received [4].
• Messages that you have deleted or moved to other folders may unexpectedly reappear in theit original folder [5][6].
• Your anti-virus software might detect infected messages that you long ago deleted, even if you emptied the Trash [7].
• It could cause problems when you try to defragment your hard disk [8].
• Your Inbox might stay blank for minutes.
• The new message count could become much larger than the actual number of new messages. A quick fix is to delete the .msf mail summary file for that folder—Thunderbird will create a new one the next time you run it—but this will not work if the folder is badly corrupted.

See also

• Why do I have nstmp folders?

External links

• Compacting and Corruption with Outlook Express
• MoreButtonsPlus extension.
• Xpunge extension.
• X-Mozilla-Status headers used to store the messages status (such as whether its deleted).