[Exim] Problem with MBX type mailboxes and Exim 3.36/IMAP-UW

Dear list

We have installed and configured Exim 3.36 on our school server where local
deliveries are saved in mbx_format.

We also have IMAP-UW installed and are testing this setup with MS Outlook
Express clients.

The error is actually reported from the IMAP server to the OE client but I
need to check all bases, so to speak. The mailbox appears to the IMAP server
as an invalid format.

The local delivery driver from the Exim configure file appears below:

local_delivery:
driver = appendfile
file = /var/mail/${local_part}
delivery_date_add
envelope_to_add
return_path_add
mbx_format
prefix =
suffix =
check_string =
group = mail

Should this work?

Using EE to view the resultant mailbox shows the distinguishing *mbx* first
line - mail is being appended as expected.

Can I assume Exim is running OK and that I should turn my attention to
IMAP-UW?