Hackers who snoop on network traffic can capture the encrypted emails that you send out. They then modify this email by enclosing the encrypted code in an image tag. When the recipient then opens the email, their client automatically decrypts the message and sends it to the hacker by requesting the image from the malicious url.
Here is the link to the website created to explain everything: https://efail.de/
Here is a link to the website announcing the discovery: http://www.bbc.com/news/technology-44107570