You would have to parse the Internet message headers to look for that.
Depending on the sending server you might see a header something like
"Content-Language". That can vary though.
Except for Outlook 2007 using the new PropertyAcessor object the object
model doesn't expose the headers. You have to use a lower level API such as
CDO 1.21 or Redemption (www.dimastr.com/redemption) or Extended MAPI for
that. Since you don't say what version of Outlook or language or platform
you're using it's hard to say anything more.
You can also look at the PR_INTERNET_CPID property, which is set based on
the received message MIME headers. It is also set on teh outgoing messages
or messages received form another Exchange mailbox even if they do not have
MIME headers.
