Encrypted email sent from Outlook 2010 cannot be read on other email clients such as Thunderbird

When you send encrypted email using Microsoft Outlook 2010, it cannot be read by non Outlook email clients such as Mozilla Thunderbird or Microsoft Entourage. There is no problem decrypting the email using Microsoft Outlook 2007 or Microsoft Outlook 2010.

This behavior also happens when a simple IMAPInternet Message Access Protocol - One of the two most commonly used Internet standard protocols for e-mail retrieval, the other being POP3. account is used in Microsoft Outlook 2010 with default S/MIMESecure/Multipurpose Internet Mail Extensions - Email protocol based on SMTP used to digitally sign and encrypt messages. settings. It is possible other email clients may have problem decrypting such emails apart from Microsoft Entourage or Mozilla Thunderbird.

There is a knowledge base article available on Microsoft site where you can find detailed description and a possible workaround for this problem.