suite script -author on the case record

I was told it is possible for a contact to be an  author on the case record? When I run my script,  I got error "The author internal id or email must match an employee". Anyone have luck on that? Or other way to work around to update case message's author without use nlapiSendEmail funcation? Thanks...

Here is my code:

var subject = datain.casenumber + "- updated";
var records = new Object();
  records['activity'] = datain.caseinternalid;
records, attachments);
nlapiSendEmail(2468, 12345, subject, datain.message, null, null, records, null); // 2468  is a contact record