Whenever you add a link to a prior email message, it will contain the parameter of "MessageViewer". This parameter will automatically remove the msg_id of your prior email and replace it with ID of the email message you just sent. To work around the "MessageViewer", you cand add this parameter, &donottrack=true, to override it.

Your message link would then look like: sitedoamin/site/MessageViewer?em_id=xxxx&donottrack=true