To access email information that is not part of body (~ or attachments, separate discussion worthy) you need to look into email headers.
For OSB Proxy polling emails these come as part of inbound.
See $inbound/ctx:transport/ctx:request/tp:headers/email:* that has values for To, From, Date, Cc, Bcc, Subject, Content-Type, Reply-To, Reply-Address and any custom user headers.