You need to check and post the exact errors that you see in /var/log/exim_mainlog. Most commonly it's a DNS resolution problem or the affected domain isn't listed in /etc/localdomains and is in /etc/remotedomains instead.
Hi Chirpy, thanks for your reply.
The domain is listed in /etc/localdomains, not in /etc/remotedomains , and there is no error reported in the exim log, which led me to take a closer look at my mail client.. there was a misconfiguration there, which is odd since I hadn't changed anything on it since sending my last successful message. Possibly Apple's 10.5.7 system update?
Thanks again for responding to my query.