It looks like it should be possible from this thread on the exim user's list: http://tinyurl.com/9xfz2 but I'm not familiar enough with Exim to say for myself. The answer to Q036 on the Exim FAQ looks interesting too. http://exim.trexle.com/exim-html-4.10/doc/html/FAQ_3.html#TOC93 Either way it seems to require a bit of hacking..