We use Exchange 2003 with Outlook 2003 clients. I have a very strange problem. When I send a message with many attachments (in this case 14) and I send them to many recipients some recipients does not receive some attachments. Al my internal users receives the attachments correctly. But my external recipients not. And it is the same mail! Some recipients receives 4, some 5 or 6 and so on.
Do you get any NDR when they dont recieve your emails. Paste the NDR here. Make sure there exchange can allows that big attachment (send and recieve limits setings for emails.). Also check for the AV running on the server and any smart host that you have?
Whether all the external recipients didn't receive the full attachments? As Rajnish asked, did you receive any NDR or error in the Application event log?