OpaqueMail OpaqueMail

Secure .NET email library and proxy with full support for SMTP, IMAP, POP3, and S/MIME. Free and open source.

It's time for secure email

For Developers
Download the OpaqueMail .NET library.

Source code on GitHub

Version: 2.0.7
Date: 2014-11-29

Install from NuGet

Download as a zip file

Protect Your Email
Learn how to encrypt your email in
3 easy steps.


Encryption Tutorial

OpaqueMail is a free .NET e-mail library with full support for IMAP, POP3, and SMTP. It provides S/MIME message signing, encryption, and decryption to foster better e-mail security and privacy.

The library follows IETF standards, implementing all IMAP4rev1, POP3, SMTP, and S/MIME 3.2 commands plus common extensions such as IDLE. It supports MIME, Unicode, and TNEF encoding.

OpaqueMail includes a fully-featured test client that allows browsing and searching of IMAP and POP3 messages as well as sending of SMTP messages with encryption. It automatically embeds images into Text/HTML messages and strips Script tags.

It inherits from System.Net.Mail.MailMessage and System.Net.Mail.SmtpClient for simplified upgrades of existing code. OpaqueMail implements .NET 4.5 async and await.

OpaqueMail is designed to be the best .NET email library. It's thoroughly documented and optimized for security, usability, and performance.

OpaqueMail is licensed according to the MIT License. It was created by and is maintained by Bert Johnson of Bkip Inc.

Where we are and how we got here

Email was invented more than 40 years ago, before network security or privacy were common concerns. The original creators could never have predicted it would become the world's most popular communications medium.

Today, billions of people spend hours reading and composing emails every day. These range from personal messages to critical business communications. Everything travels through email, including sensitive information.

The overwhelming majority of emails are sent in "plain-text", meaning that everybody involved in sending your message can (and does) read it. And that's significantly more people than you'd expect.

The problem's even more serious when using an untrusted network, like the neighborhood coffee shop. Free tools allow anyone to snoop on unencrypted email traffic. That's unacceptable.

Whatever you have to send, an email's audience should be limited to you and the recipient. Not some technician at your ISP, not the government, and certainly not a hacker on your wifi.

The solution already exists

"Public key cryptography" (a.k.a. asymmetric encryption) is the answer to secure messaging. This is the underlying technology behind SSL and TLS, which are used for everything from banking to military security.

Two standards have existed for mail encryption since the 1990s: S/MIME and OpenPGP. Both approaches work by encrypting email in a way that only the recipient can read. First, a "public key" is used to compose a secure message. Then, the recipient uses a "private key" (like a decoder ring) to decode the message.

Both OpenPGP and S/MIME are free, proven, and secure.

Let's fix that

The OpaqueMail project's goal is to normalize the use of email encryption. The first step is education, but we ultimately seek to make secure email the default option.

  1. Let's get the word out.

    First, spread the word about why email privacy matters and what we can do about it. We live in a critical time for defining and protecting our personal liberties online.

    We all deserve freedom of speech and confidence that our communications are secure.

  2. Let's start signing and encrypting our messages.

    It only takes a few minutes to get started.

    Check out our email encryption tutorial now. Make sure to set up encryption for all of your devices that support it and encourage your friends to do the same.

    Lead by example. And consider updating your email signature to link to http://opaquemail.org/ to spread to word.

  3. Let's create better software.

    Going forward, all mail applications should not only support S/MIME, but encourage users to default to it. It shouldn't be a hidden, arcane option limited to power users.

    To support that end, we've created a free open-source .NET email library with full S/MIME support via IMAP, POP3, and SMTP available on GitHub and NuGet. We'll share other resources for simplifying S/MIME using other programming languages.

    If there's enough community interest, we'll also create secure email clients for Windows, Mac OSX, Linux, iOS, and Android.

What's in a name?

Headlines in 2013 were dominated by "Prism", the US government's clandestine surveillance program. A prism refracts light, such as the fiber optics underlying the internet, allowing true colors to be studied individually.

"Opaque" is the opposite of "transparent", meaning that it's impenetrable to light. Opaque objects can't be refracted. OpaqueMail is resistant to Prism.

.NET email library and proxy available according to the MIT License

Project founded and maintained by Bert Johnson of Bkip Inc.

Bert at OpaqueMail dot org