SetFieldValue of 'email' text box in Invoice

I'm having trouble getting a script to work.  I want to set the value of the text box 'email' on an invoices' communications tab.  I have a script that should be pulling the value from another field located on the invoice (internal ID of custbody40).  When I save the invoice, it should take the value in the custom field and populate the email text box, though it doesn't seem to be doing that.  Any help would be greatly appreciated.

function userEventAfterSubmit(type, form, request){      var fldEmail1 = nlapiGetFieldValue('custbody40');      try{          if(fldEmail1){              nlapiSetFieldValue('email', fldEmail1);          }       }      catch(error){          if (error.getDetails != undefined)              {                 nlapiLogExecution('ERROR', 'Process Error', error.getCode() + ': ' + error.getDetails());                 throw error;             }             else              {                     nlapiLogExecution('ERROR', 'Unexpected Error', error.toString());                 throw nlapiCreateError('99999', error.toString());             }      }  }