ACME Email Server

CASTLE Platform® offers an ACME Email server that implements the Email-Reply-00 challenge defined in the draft of ACME next version.

Free S/MIME certification

Get an open-source ACME email client based on Certbot for automatic & free S/MIME certification. Get your emails secure.


Why free S/MIME certificate

Thanks to Let’s Encrypt project, Internet is a more secure place. Aiming at following the same philosophy, the next step is to secure e-mail communications. With our ACME server, we are ready to issue certificates for S/MIME signing and encryption, in a 100% automatic way. Our vision is continue the path opened by HTTPS to email communications. And completely free to get everyone.

Securing email communications

How are we different?


We are a national R+D public center, with more than 20 years of consolidated experience in communications. We have the knowledge to deploy the latest improvements on the state-of-the-art technologies. And we do it.


We are not enterprise, we are a public foundation. We push the edge of the knowledge to make the world a more reliable, trustworthy and secure place. Our vision is to get everyone, users, small and big companies.


Our team is composed by Ph.D., engineers, Senior researchers, PHY & Network researchers from many nationalities. We embrace the progress by integrating different approaches and ways of thinking.

Get a free S/MIME certificate

We developed an open-source ACME client based on Certbot able to generate and accomplish the ACME challenge and get a PKCS12/PFX certificate, ready for your email client. This ACME email client is able to issue a S/MIME PFX certificate without no interaction and 100% completely automatically.

Furthermore, With a S/MIME certificate your emails can be signed and verified. Plus, you can also sign documents or PDF.