Load Balance SMTP for Exchange 2007 relay

This isn't an exchange problem or an LTM problem, it's a limitation with the smtp protocol in that there isn't support for an x-forwarded-for header like http has. So your choices are to a) set the LTM as your default gateway on your mail servers b) bridge the traffic through LTM or c) insert the client IP in some sort of mail header that you can then configure your mail server to read, act, and sanitize on.