Outlook Express and SBS 2003

I have just upgraded from Exchange 5.5 to SBS 2003. I have two remote clients that use Outlook Express and POP3 to get their email from the exchange server. It worked fine to download mail but they cannot send. When they try to send it brings up a dialog box with the old domain in it. It is greyed out so they cannot send. Until I can migrate them to a VPN solution I need this to work. OWA might be a solution but then they don’t have their contacts.
What puzzles me is that they are authenticating correctly to receive mail but sending is a problem. Please help.
Thanks,
Chris