When using IMAP, Apple Mail.app can occasionally have problems displaying your messages properly. This occurs when the cached IMAP messages on your computer become corrupted. There are two ways to solve this.
Rebuild The Mailbox
1. Select the mailbox that is exhibiting the problem in the mailbox list of the Apple Mail main window.
2. Click the Mailbox menu item in the menu bar at the top of your screen.
3. Select the Rebuild item to rebuild the mailbox. This can take a long time depending on how much mail is in your mailbox.
If this does not work, you can use the method below to do clear the Apple Mail IMAP cache and force Apple Mail to re-download all of your email from the IMAP server.
Clear The IMAP Cache
1. Quit Apple Mail. It cannot be running for this procedure.
2. Click the Finder icon in your Dock. This will open a new file browser window.
3. Go to the Library folder.
4. Go to the Mail folder.
5. Find the folder named IMAPfirstname.lastname@example.org. email@example.com should be the email address that you are having problems with.
6. Move this folder to another location on your hard drive. Do not delete this folder in case there is a problem and you need to restore it.
7. Open Apple Mail and it will now re-download all email from the IMAP server and rebuild your IMAP cache. This can take a while depending on how much mail is in your account.