You'll encounter that problem a lot; more and more mailhosts are refusing to talk with dynamic IPs and hosts that don't reverse-resolve properly.
To that end:
[link|http://www.exim.org/exim-html-4.40/doc/html/FAQ.html#TOC124|http://www.exim.org/...l/FAQ.html#TOC124]
304 is the Q of interest.