Linux not delivering over NFS

I have mounted /var/spool/mail from another system. When exim tries to
deliver a message into that mounted NFS volume I get the following
messages:

1996-12-23 11:08:13 clameter@???: local_delivery transport
deferred: failed to lock mailbox /var/spool/mail/clameter (lock file)
1996-12-23 11:09:34 clameter@???: local_delivery transport
deferred: failed to lock mailbox /var/spool/mail/clameter (lock file)

There is no lock on that mailbox as far as I can tell.

And exim leaves a lock file(?)

[hur]~:l /var/spool/mail/clameter*
-rw-rw----   1 clameter mail            0 Dec 23 10:56 /var/spool/mail/clameter
-rw-------   1 clameter mail            0 Dec 23 11:22 /var/spool/mail/clameter.lock
[hur]~:

I am running Linux 2.0.27 under Debian 1.1 on the Server and Debian 1.2 on
the NFS client.

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PGP Public Key = FB 9B 31 21 04 1E 3A 33 C7 62 2F C0 CD 81 CA B5