Internal & External email accounts

I have SBS 03 with Exchange with a .local domain. Internally i have no problem sending email in and outside. Also we have a different domain name .com with emails hosted by an outside company. In outlook each user has 2 account: exchange server and POP for external account. Everytime someones sends an email using his external account to an internal account it will receive an NDR report and emails fails. Of course, ih he switches the accounts and sends an email using internal it works great. My question: is there someting that I can do in our Exchange so that emails can be delivered?
Thank you.