From Pixel Transitions:
CryptWit is a Free data encryption and data compression add-in for Outlook. It is designed for Outlook versions 2007, 2010 and 2013.
The add-in can encrypt message body as well as any number of attachments up to 20MB total.
Recipients do not require to have CryptWit to decrypt and read email messages.
With CryptWit you can send private messages without fear of being read by an unintended recipient. With CryptWit your colleagues or IT support will not be able to read your secure email. CryptWit encrypts the message in the computer memory before it's sent out and saved into your Sent folder. The original message is stored in your Sent folder in the encrypted form. If anyone tries to open your Sent mail in your absence, they won't be able to read CryptWit'ed messages without the correct password.
It's all even easier on the recipient's end. They don't have to install any additional software. Your message will arrive in the form of a standard encrypted ZIP file. They will be prompted to enter the password once they try to unpack it. You should tell the password to the recipient, or send it to them separately, as it doesn't come in the same message.
CryptWit add-in for Outlook is completely Free. It does not require any registration, has no limitations requiring you to purchase any license or subscription, has no fees whatsoever.
CryptWit does not communicate to any web servers, in fact, it does not even require an internet connection. The only requirements are Microsoft Outlook 2007-2013, and Microsoft .NET Framework 3.5 or later. The product comes as a digitally signed setup package which should detect all dependencies and install required components, including the Framework.