DLL GetErrorString(ErrorCode) example:

extern void SetDebugOn (void) ".\\EMAIL100.DLL";
extern int EmailCmd(int,int,char*) ".\\EMAIL100.DLL";
extern char* GetErrorString(int) ".\\EMAIL100.DLL";

...
SetDebugOn();
var nInfo = new netInfo();
var Handle = nInfo.sessionID;
var ErrVal = EmailCmd(Handle,NewEmail,"");
if (ErrVal < 0) {
  var ErrMessage = GetErrorString(ErrVal);
}
...