host name lookup failure

[andyleftylefty]

thread14-1217101

I have a FreeBSD 5.4 box running Sendmail 8.
I had a remarkably similar problem to one in the referenced thread (sendmail emails won't go out, "host name lookup failure" entries in the log).
I resolved the issue by replacing the "Smart" relay host entry, which had been the DNS name of the relay server, with the IP in brackets of the relay server:
DS[x.x.x.x]

This worked like a charm, but my question is why?
Sendmail had been working fine using the DNS name of the server and then all of a sudden it won't send unless it has an IP as the relay host. I know I'm missing something.

Curiously, I was able to ping the DNS name, do an nslookup, even send a TELNET email via the relay server. But sending from sendmail would dump the deferred email in the queue and add that "host name lookup failure" entry in the log.
/etc/hosts and /etc/resolv.conf had valid entries, as far as I can tell and I even set up a mailertable.

If someone out there can explain this to me I would greatly appreciate it.

Thanks!

Best,

Andy