Why fstat() in qmail-send.c:markdone()?

[Fred Lindberg]

It's obviously important or it wouldn't be there. Can anyone explain
why?

>From qmail-send.c: markdone():

 for (;;)
  {
   fd = open_write(fn.s);
   if (fd == -1) break;
   if (fstat(fd,&st) == -1) { close(fd); break; }
   if (seek_set(fd,pos) == -1) { close(fd); break; }
   if (write(fd,"D",1) != 1) { close(fd); break; }
   /* further errors -> double delivery without us knowing about it, oh
well */
   close(fd);
   return;
  }

Thanks!

-Sincerely, Fred

(Frederik Lindberg, Infectious Diseases, WashU, St. Louis, MO, USA)