Images appear as a question mark in mail and safari
I have just noticed a new problem in Mail and Safari. "Some" emails that contain pictures that are links are suddenly only showing the images as a small blue square with a question mark. If I click the "view this email in your browser" link in Mail it opens in Safari but with the same problem! Then if I paste the URL into Firefox or Chrome the email is displayed correctly. The email is also displayed correctly on my iPhone 6. I really can't figure out what has changed as it is has effected emails that are filed and that displayed correctly before. Any help appreciated. Many thanks Phil.
MacBook Pro, OS X Mavericks (10.9.3)