Mail not displaying message body content The Mail app on my Mac stopped displaying message body once the message is opened. 

I can still see the message body in the message preview in each mailbox, but when I click (or double click) on a specific message, the message body is blank in the new window. 



This happened to me a while ago, some time after updating to El Capitan. It affects all my email accounts on my Mac, Mail works properly on my iPhone. I don't remember changing any settings.

How to resolve it?
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.