Hi,
Follow the below steps to when exchange can't send email Using POP3 Connector and Small Business
Server 2003:
To help determine which stage or stages of e-mail delivery are successful, follow these steps:
1. Start the Server Management tool. To do this, click Start, and then click Server Management.
2. Expand Advanced Management, expand Computer Management, expand Services and Applications, and then click Services.
3. In the right pane, right-click Simple Mail Transfer Protocol (SMTP), and then click Stop.
4. In the left pane, click POP3 Connector Manager, and then click Open POP3 Connector Manager.
5. Click the Scheduling tab, click Retrieve Now, and then click OK on the following message that appears:
The Microsoft Connector for POP3 Mailboxes will now download and deliver e-mail. This may take a few moments.
6. Click OK.
7. View the contents of the PickUp folder to determine whether files appear. These files are the e-mail messages that the POP3 Connector retrieves from the remote POP3 server. These files have a .eml file name extension. To view the contents of the PickUp folder, click Start, click Run, type
%programfiles%\exchsrvr\mailroot\vsi 1\pickup, and then click OK. Use a text editor, such as Notepad, to open these files. Verify that these files are valid e-mail messages and determine the recipient of each e-mail message.
Source: http://support.microsoft.com/kb/885685
