How to rebuild the mailbox index in Apple Mail

The Apple Mail application keeps track of all emails by means of an index stored locally on your computer. This, among other things, allows for fast full-text search in all messages. For various reasons it may happen that this index gets corrupted.

Typical symptoms:

  • some attachments are displayed as binary code and cannot be opened
  • when clicking on one email, the message of a different email is displayed

In most cases the issues can be fixed by telling the mail client to rebuild the mailbox index:

  • Select the folder with the broken emails (Inbox or a subfolder)
  • In the Mailbox menu, click on the last entry Rebuild

Depending on the amount of emails in the folder, the rebuild process can take from a few seconds to a couple of minutes. Once it's finished, your emails should again be displayed correctly.