A mail delivery puzzle

[bill at horne.net (Bill Horne)]

Thanks for reading this.

I have a ham radio domain name, namely "w1ac.ampr.org". The ampr.org domain
is used by ham operators to lookup addresses in the 44.0.0.0/8 space, which
is a semi-detached network connected via amateur packet radio.

Now, the problem: I have an MX record in the ampr.org dns, which points to
billhorne.homelinux.org, the dynamic dns domain name for my Linux firewall.
If I send email from either netscape.net or hotmail, to
<user>@w1ac.ampr.org, it gets through OK. I have set the new domain in the
sendmail.rc file, so sendmail knows it's supposed to take mail for
w1ac.ampr.org (and indeed it does, from hotmail/netscape). However, mail
from my attbi.com address produces an error from the attbi.com postmaster,
saying that "Host w1ac.ampr.org not found".

Does anyone know why this is so?

Ideas? Workarounds?

TIA.

Bill Horne