Mail (Yosemite): If messages aren’t displayed or appear incorrect

Display the message in another format. Choose View > Message > Plain Text Alternative. The alternatives available depend on the original format of the message.

Rebuild mailboxes.

For a POP account, reimport mailboxes. Choose File > Import Mailboxes, then select Apple Mail. In the Import dialog, display the Library folder (hidden by default) in your home folder by pressing Command-Shift-G, then entering ~/Library. In ~/Library/Mail/V2, select the folder that begins with “POP-” (for example, POP-eharrold@mail.example.com).