You are probably safe
ProtonMail is secure and your data is safe. Sometimes your browser may show you a warning (the exact message depends on the browser you use) that says something along the lines of “Connection is Not Secure”. This can happen for a variety of reasons but the most common one is due to loading an email with insecure images.
Most emails today contain images. ProtonMail uses SSL/HTTPS to secure your data, but some emails can load images using plain HTTP. The following code is an example of such an email:
Hello there, We have a new promotion: <img src="http://site.com/promo.jpg"> Check it out!
Because ProtonMail is secure with HTTPS and the image is loaded using HTTP the browser warns you that not 100% of the page is secure.
For now there’s not much you can do to avoid these warnings aside from not loading any images at all. A permanent solution is for ProtonMail to host all images, but this requires some significant effort in order to keep your data end-to-end encrypted and won’t be coming out in 2016.
You can read more about this warning message in the following technical article:
The other reason why you would get this message is if our SSL certificate is compromised (although this has never happened and is unlikely since it requires the Swiss government to actively participate in such an attack). The warning message you get from your browser should detail the cause. If you’re worried about such a case, you can always contact our security team for help.