[exim] "Retry Timeout Exceeded" - Mail not getting out

I have recently set up Exim4 (4.44-2) (exim4-daemon-light) on a Debian
Testing/unstable box, with setup done through "dpkg-reconfigure
exim4-config". The server is on a box on a DSL line, with a static IP, and
its own domain name.

I can receive email without any trouble, and can send emails out to any
email account at the ISP providing the DSL service. Any other emails sent
from the server, however, are never received. I also don't seem to get any
notification that they didn't get through (though I'm not entirely sure
where this information would be sent); the only log entries are in
/var/log/exim4/mainlog.

Here is what /var/log/exim4/mainlog shows for a message sent to (sample)
email address user@??? (I've replaced IP addresses with 0.0.0.0):
___________________________________
2005-02-16 12:13:15 1D1Sjf-0005qI-Jz <= root@??? U=root P=local
S=276
2005-02-16 12:16:25 1D1Sjf-0005qI-Jz mailserver.example.org [0.0.0.0]:
Connection timed out
2005-02-16 12:19:34 1D1Sjf-0005qI-Jz mailserver1.example.org [0.0.0.0]:
Connection timed out
2005-02-16 12:22:43 1D1Sjf-0005qI-Jz mail.example.com [0.0.0.0]: Connection
timed out
2005-02-16 12:25:52 1D1Sjf-0005qI-Jz mail.example.com [0.0.0.0]: Connection
timed out
2005-02-16 12:29:01 1D1Sjf-0005qI-Jz mail.example.com [0.0.0.0]: Connection
timed out
2005-02-16 12:32:10 1D1Sjf-0005qI-Jz mail.example.com [0.0.0.0]: Connection
timed out
2005-02-16 12:35:19 1D1Sjf-0005qI-Jz mail.example.com [0.0.0.0]: Connection
timed out
2005-02-16 12:38:28 1D1Sjf-0005qI-Jz mail.example.com [0.0.0.0]: Connection
timed out
2005-02-16 12:38:28 1D1Sjf-0005qI-Jz == user@??? R=dnslookup
T=remote_smtp defer (110): Connection timed out
2005-02-16 12:38:28 1D1Sjf-0005qI-Jz ** user@???: retry timeout
exceeded
2005-02-16 12:38:28 1D1Sjf-0005qI-Jz Completed
____________________________________

The remote server is definitely up and running. The email is never
delivered. I also tried sending mail as a user, same problem.

I tried sending emails to a number of other email accounts (including
yahoo.com and hotmail.com); no messages get through to any of them. I am
guessing that remote servers doesn't like something about the emails my
server is sending, or don't like my server's replies to some lookup call,
and is silently dropping the emails I am sending as spam. I checked to see
if my IP address or domain name are blacklisted, and they are not.

I am assuming I have missed some basic configuration detail, but have looked
through the Exim FAQ and documentation and couldn't figure out where to
begin.

Any suggestions or hints as to what things to check would be welcome.

Thanks,

Bruce