Mail not displaying message body content
Rebuild the Mailbox

Try rebuilding the affected mailbox:


Select the Inbox or affected mailbox.
Select Mail.app > Mailbox (menu) > Rebuild


Rebuilding the mailbox will cause Mail.app to read all the messages again from the server. Rebuilding often fixes problems and restores missing or malformed messages.

From Apple's support document Mail: Rebuild mailboxes:


  You may need to rebuild a mailbox to update the list of messages it contains. For example, if messages appear to be missing or garbled, or if you can’t find all relevant messages when you search using the Entire Message search option.