[cobalt-users] raq2 - dns/mail problem? "host must resolve"

[David Liu <dyliu@xxxxxxxxxxx>]

hi folks-
i just sent an email to someone in finland (e.g. domain.fi) and it got 
bounced back for the following reason....

Your message cannot be delivered to the following recipients:
Recipient address: someones@xxxxxxxxxxx
Reason: Message rejected; bad return address.
Diagnostic code: smtp; 551 <me@xxxxxxxxxxxxx>... host must resolve

what happened? this has never happened before!
below is what i have under the sysadmin page for my domain

domain.com > 123.123.123.123
www.domain.com > 123.123.123.123
123.123.123.123/24 > domain.com
123.123.123.123/24 > www.domain.com
domain.com     mail is sent to High priority mail server     www.domain.com

thanks in advance for your help!
dave