OE Disconect errors

I've configured a Courier IMAP / LDAP / QMAIL server for a firm with 2
offices connected via an ISDN dedicated circuit. Both offices have DSL
connections to the Internet. The primary office hosts the server and has no
problems. Both offices use Outlook Express 6

Secondary office gets constant disconnect messages when using OE. The end
user clicks OK, OE reconnects & everything is fine.

Any idea what might be causing this problem?

I've tried changing the timeouts on the OE clients & having the secondary
office come thru the private (ISDN) network.