Outlook Connectivity Problem

I have Exchange Server 5.5 SP3 installed on NT Server 4 SP6a. I had a problem with the MTA service hanging up which I resolved after a struggle. Now all my XP and Win2K Outlook 2000 clients are connecting to the Xchg server fine, but my Outlook 2Kclients on NT 4 SP6a boxes either won’t connect at all, or do so much more slowly than they did before. I’ve tried the obvious (rebooting the client), the less obvious (uninstall/reinstall Exchange Server as a Service in the Outlook client) and thedesparate (running SP6a again), all to no avail. Any thoughts/suggestions are greatly appreciated.