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109: Why does my email program report an incorrect number of messages in my Inbox, or other folder?


All computer programs exhibit eccentricities at times. This is one of the most common: “Thunderbird [or other email program] says I have 27 emails in my Inbox, but I can see that I actually have 43. What’s going on?”

It turns out that email programs can get confused after long use. Email Inboxes and folders — like those used by programs from Apple, Mozilla, and Microsoft — may actually contain both new email and deleted email, though the latter is hidden by the deletion process. Until an email folder is “compressed,” or “compacted,” or “purged” (depending on the terminology used in the program), “deleted” emails will be hidden but will actually continue to fill up the folder, causing it to grow very large. At some point, the folder may becomes too large for the program to handle and errors will start to appear. Email is not lost at this point, only misplaced.

This situation is generally easy to remedy via the following procedure:

  1. Start your email program.
  2. Select the folder that is reporting the incorrect message size.
  3. Locate and click the “Compact Folders” (or similar) option, which is usually under the “File” menu.
  4. After the folder is successfully compacted, all deleted items have been removed from your email folders. (The index file used by your email program to keep track of current email will also be rebuilt correctly, fixing the “incorrect number” problem.)

If you are backing up those email folders locally you will see a noticeable decrease in their size. In general, it is good practice to compact all email folders every week or so, depending on the volume of email. In the longer term, it may be easier to set your email program to automatically compact email folders on a regular basis. If your version supports this feature, you may activate it this way:

  1. Start your email program.
  2. Locate your preferences or options settings. In Outlook it is generally labeled “Preferences” and found under the “File” menu. In Thunderbird you would find it by looking for “Preferences” or “Options” under the “Edit” or “Tools” menu (depending on your version of Thunderbird). Access to these settings may differ for other email programs, but all email applications have them.
  3. Next look for an “Offline & Disk Space” or “Network and Disk Space” option. Locate and check the box labeled “Compact folders when it will save over [xx] KB.” You may set the threshold to any amount you like. A smaller value will result in compaction more frequently; a value of, say, 200 KB will trigger compacting every few days.


At times, folder compaction does not finish correctly and results in error messages; this generally occurs only when email folders have grown extremely large. The error, in this case, may be the result of insufficient computer resources available to complete the operation. Submit a support request via the Mines Help Center ( for aid.