Digitally signed messages remain in the Messages pending submission queue and are not delivered in Exchange 2003

This is due to the Transport Scanning feature being enabled which needs to be enabled only on front end server.
Solution:

Turn off the transport scanning on the Exchange server by modifying the following registry.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\TransportAVAPI - Set to 0
Restart requirement
You must restart the Microsoft Exchange Information Store service on the Exchange computer.

Reference

https://kc.mcafee.com/corporate/index?page=content&id=KB58822&actp=search&searchid=1226840760543