Sending Attachements default to the exchange account.

I am running exchange server 2003. I am not using the exchange account but have POP3 email accounts as default. Everything seem to work fine until I try to send an attachment via mail recipient through a programl. Example: If I am working on an Excel spreadsheet and want to email it to another person I click file>send>email as attachment. It pulls up a new email window and I enter the address I want to send to. After I send I get an error from Exchange saying message was undeliverable and it says it could not send using the exchange account. I have the option when sending the email to check which account I am sending from and it is the correct POP3 account. Any help or insight would be greatly appreciated.