Question : 571 Unable to determine MX host for ...

I set up a Windows 2003 Server SBS Premium Edition with Exchange 2003 this year. I had everybody using internal (MX mail) and external email fine, then this message:

 (The following recipient(s) could not be reached: John Smith on "DATE and TIME"  571 Unable to determine MX host for "i.e. '[email protected]al'")

started popping up in the inbox from the SYSTEM ADMINISTRATOR everytime someone tried to send internal email. This started happening about 4 months after is was installed. This is when the POP3 email in Outlook 2003 is set as the default email and the Exchange email as not the default. When I switch it to having the Exhange Server email as the default, the INTERNAL email works fine, but then the external email brings the following message to the inbox from the System Administrator: The following recipient(s) could not be reached:

      [email protected] on 9/23/2005 12:04 AM
            The e-mail address could not be found.  Perhaps the recipient
moved to a different e-mail organization, or there was a mistake in the
address.  Check the address and try again.
            al #5.1.8 smtp;553 5.1.8
al>... Domain of sender address [email protected]l

What the bleepidy beep is going on here?
Verizon is our ISP.
There is a Linksys WRV54G router giving the DHCP and Firewall duties to the network
There are 2 network cards in the server, but only one of them is hooked up and it can access the internet as well has handle all internal traffic.

Please no guessing. Only answer if you truly know what is going on here. i am willing to pay $100.00 to the individual that can fix this for me.

Answer : 571 Unable to determine MX host for ...

Lets deal with the first bit...
You don't need to change the login name. After setting the email addresses simply deselect the option "Allow recipient policy to update email addresses automatically". This will stop Exchange from setting the email addresses back to the default.

Your laptop users need to migrated in the same way. When you configure them for Exchange email, enable the option to use "cached mode". This is basically enhanced Offline Folders from the previous versions of Outlook.
If you make extensive use of public folders and these laptop users need to access that data, look at the Office Resource Kit web site for the group policy settings that allow a public folder in "Favourites" to be cached as well.

Simon.