KB Article: http://support.microsoft.com/kb/2142236
When sending an encrypted message from Microsoft Office Outlook 2010 to a recipient using a third-party email client, such as Lotus Notes, Entrust, SeaMonkey, or Thunderbird, the recipient may not be able to read the encrypted message. In the case of the Thunderbird email client, it may display the following message in the body of the message when they open it:
Thunderbird cannot decrypt this message
The sender encrypted this message to you using one of your digital certificates, however Thunderbird was not able to find this certificate and corresponding private key.
The Thunderbird client may display the following warning:
Message Has No Digital Signature
This message does not include the sender's digital signature. The absence of a digital signature means that the message could have been sent by someone pretending to have this email address. It is also possible that the message has been altered while in transit over the network. However, it is unlikely that either event has occurred.
Message Cannot Be Decrypted
This message was encrypted before it was sent to you, but it cannot be decrypted. There are unknown problems with this encrypted message.
Also, Microsoft Entourage 2008 (included in Microsoft Office 2008 for Mac) and Microsoft Outlook 2011 for Mac may be unable to decrypt email messages sent from Outlook 2010. You may see the following error on Outlook 2011 for Mac:
The security of this message cannot be verified because of an error.