Odd rejection on my local mail server

[lug at the-leveys.us (Don Levey)]

This morning my Linksys BEFSR41 finally decided to give up the ghost.  Well,
first it got senile dementia and forgot what was going on about every 10
minutes.  However, as this was 6 years old I wasn't to upset.  The model is
still current (now on version 4) so I picked up another one and set it up.
But now I'm having odd mail rejection problems on my local server.

In particular, I'm having difficulty sending mail outbound.  I get the
following message:

   ----- The following addresses had permanent fatal errors -----
(my sister)@aol.com
    (reason: 550 5.7.1 <(my sister)@aol.com>... Relaying denied. Proper
authentication required.)
    (expanded from: (my sister)@aol.com)

   ----- Transcript of session follows -----
... while talking to davinci.the-leveys.us.:
>>> DATA
<<< 550 5.7.1 <(my sister)@aol.com>... Relaying denied. Proper
authentication required.
550 5.1.1 (my sister)@aol.com... User unknown
<<< 503 5.0.0 Need RCPT (recipient)


I was able to get around this by adding my external IP to the relay-domains
file, but I shouldn't need to do that.  It's always worked before; the only
two things that changed are the router and the external IP.

Question: since the change happened tonight, is it possible that the old IP
is still in a cache somewhere?  If so, then my domain *name* would still be
resolving to the old IP, and not matching to the new incoming address - thus
bouncing, no?

 -Don