SMTP mailing problem.

Trying to get a SMTP server running on my DSL-connected box at home. I've got a dynamic domain name, which is properly configured right now. It will connect you to Apache/ftp/telnet/whatever just fine.
I'm trying to get it so I can send e-mail directly from the same box, but whenever I try to send anything out, it doesn't go, and says "no route to host" in the mailer logfile. Traceroute, ping, and everything else I can think of come back working, though.
I tried to send a message from my Yahoo e-mail account to my server box, and it bounced right away with this message:
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:
64.228.193.24 does not like recipient.
Remote host said: 550 relaying to prohibited by administrator
Giving up on 64.228.193.24.

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I used the IP, rather than the domain name, because I thought maybe it was a DNS error, or something like that. Doesn't seem to make any difference, though.
I'm using Debian Linux, Exim mailer, the regular mailx mail client, smtp is enabled in my inetd.conf file, and I'm completely out of ideas. Anybody else got any?