I'm sure that other people have had this same problem, but I haven't yet been able to find a good solution for it.
We are running Exchange 5.5 SP4 with all of the latest hotfixes (as of 3 weeks ago anyhow).
When remote users connect to the Exchange server using Outlook in MAPI remote connection mode (cached Exchange server mode in Outlook 2003, Offline folder (.OST) mode in previous versions) and try to download a message with a large attachment, their Outlook basically stalls and takes far longer to download the email than it would using POP3.
The users are connected to the internal network using Netsecure VPN software and our firewall is a Netscreen 10.
I don't want to open the holes in the firewall necessary to connect to the Exchange server over RPC because that is a very big security risk. In a few months, after I get everything moved to Active Directory and Exchange 2003, this won't be an issue because of RPC over HTTP, but until then it is an issue.
I have tried the methods suggested in the Technet Document: MS Exchange Server and Outlook: Optimizing Messaging Client Performance, but that hasn't solved the problem.
I see some other things to try in this document about using Outlook 2003 in Cached Exchange mode, but if anyone has any other ideas, please let me know.