If you are using the POP3 connector to receive mail, but you have some accounts (like a remote user) that are not collected by the SBS box you will find that it’s not possible to send mail to those accounts from inside the SBS domain.
As soon as you try to send mail to that account, Exchange will send you and NDR back saying “The email account does not exist at the organization this message was sent to. Check the email address, or contact the recipient directly to find out the correct address.”
This happens because by default Exchange is setup to be authoritative for the domain, any address that doesn’t exist on you server will appear as undeliverable. The best way to workaround this is to collect all mail on your SBS box and then make the remote user connect to the SBS box instead of your ISP (you could use OWA, RPC over HTTP or even POP3/IMAP).
Another workaround to this problem is to open Exchange System Manager-> right-click Virtual SMTP server-> Properties-> Messages tab-> Specify your ISP’s SMTP mail server on the field “Forward all mail with unresolved recipients to host”.
Note: If you are using SMTP to receive mail you should not do this (there is no need anyway)… you could cause a mail loop because of this.