MS Exchange 2003 & Pop3

Hi,
Our organisation uses MS exchange 2003 internally and Pop3 to send/recieve external email. We never used to have to change from the Exchange Account to the Pop3 account to send external email, it used to pick up that it was external and use the correct account. Ever since we have Outlook 2003, we have to change accounts to send internal/external email otherwise we get a delivery faliure message. Is there anyway to have it back to the way it was where you didn't have to keep changing between the Pop3 and exchange account all the time? Any help would be greatly appreciated. Thanks,

Charlie...