Google announced yesterday of new feature in Gmail and Google Mail for Business, you can read it from their blog post http://gmailblog.blogspot.fi/2013/12/images-now-showing.html by Senior Product Manager John Rae-Grant.
What happened: Google started fetching all external images in emails, scan them for known viruses and malware (trojans). This has already started on desktop users, and it will be roll-out to mobile early 2014.
Good results: Most HTML-formatted email will look better, ie. email users don't have to "trust" the sender to see the pictures. User privacy is enhanced.
Bad results: Email marketeers will not be able to report how many Google users (recipients) opened the email, ie. viewed email with pictures, since Google will fetch all pictures, your reports will display 100% open rate for Google Gmail and Apps for Business users. It will mean that companies wanting to learn how to communicate better with their customers, are not getting all the relevant feedback anymore.
Software vendros, EDM and Marketing Automation dominantly, will be updated accordingly, maybe filtering Google users out of the reports pertaining open success indicators, so it doesn't skew the reports. You can still have an estimation of how effective your email sender / subject was, ie. how many have viewed the email with pictures.
Open indicator has never been a very accurate number, but Google just made it even less accurate.
Next eyes will be focusing to Microsoft (Hotmail, Outlook.com), Yahoo and AOL, will they follow suit?