ProtonMail, a secure email service provider, has been under siege of a sustained enterprise-level  DDos attack that has left the service unavailable since Tuesday. ProtonMail said the attack is from unknown parties who have flooded their IP addresses, in a company blog.

“That quickly expanded to the datacenter in Switzerland where we have our servers,” ProtonMail said on its blog.  In the process of attacking us, several other tech companies and even some banks were knocked offline temporarily,” the company added.

The company has been unable to stop the attack so far, despite its best efforts, and say they are working to restore normal service. Despite the current limited service, the core end-to-end encryption, which is the company’s main draw, remains “100% untouched,” according to the information on the blog.

ProtonMail offers a webmail system designed by boffins and CERN to withstand surveillance by the world’s intelligence agencies. The company is working with experts in their Swiss data center to try and resolve the issue.