[Exim] Windows 95 TCP configuration oddity.

We use Pegasus (3.12b Win32) as our client, running on Windows 95.

Recently, I've noticed that if the Microsoft TCP stack is setup manually,
rather than our usual DHCP settings, Exim rejects attempts to send mail.
Pegasus reports a remote protocol error, and Exim adds an entry to
exim_rejectlog stating:

rejected HELO from [10.1.50.5]: syntactically invalid argument(s): [1681]

If I set the TCP stack back to defaults, remove any current registry
entries for the TCP stack and reboot, the PC picks up its settings via
DHCP and everyone is happy.

Any ideas why Exim is unhappy with the fixed setup ?