Originally Posted by GrayHare
That hover popup is not universal, instead it's a feature of whatever email viewer you are using. In most email viewers it is safe to view the text of an email; attachments and links are where malicious content can hide. Note that if you allow your email viewer to display pictures, the pictures can reveal to the sender that you have seen the email, along with information about when and where you did so.
Good advice. I've restricted my email for years to not allow pictures to be shown unless I specifically authorize it (each one has a little button at the top that says "Load Images").
Also be aware that you might not even know that there was a picture in the email, because it can be just one pixel in size.