In Microsoft Outlook for Mac 2011 (version 14.6.7), which is part of the Microsoft Office for Mac 2011 office suite, while composing an email message, you can click on Draft on the top menu bar and then select Security to choose to digitally sign and/or encrypt a message to the recipients.
If you don't have a signing certificate installed for the account from which you are sending the email, though, when you click on Send, you will see the message "You must first install a valid signing certificate before you can send a digitally signed message. Do you want to send this message without a digital signature?"
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