What does Microsoft Exchange Intelligent Message Filter (IMF) do with email that's been signed or encrypted using the Secure Mime (S/MIME) standard?
By design, IMF ignores such email. The tool can't read S/MIME-encrypted messages; furthermore, the fact that the message was encrypted to a particular recipient is considered reasonable evidence that the message is legitimate. For the same reason, the tool doesn't scan signed messages.
—Paul Robichaux