[exim] missing log entries

Hello,

we are running an exim4 mail server (debian squeeze) release 4.72.

exim -bV says:

Exim version 4.72 #1 built 25-Oct-2012 18:35:58
Copyright (c) University of Cambridge, 1995 - 2007
Berkeley DB: Berkeley DB 4.8.30: (April 9, 2010)
Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc GnuTLS
move_frozen_messages Content_Scanning DKIM Old_Demime
Lookups: lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmnz
dnsdb dsearch ldap ldapdn ldapm mysql nis nis0 passwd pgsql sqlite
Authenticators: cram_md5 cyrus_sasl dovecot plaintext spa
Routers: accept dnslookup ipliteral iplookup manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp

log_selector (from exim4 -bP) is:
log_selector = -retry_defer -skip_delivery -host_lookup_failed
-lost_incoming_connection -rejected_header +received_sender
+received_recipients +sender_on_delivery +delivery_size +queue_time
+deliver_time +queue_time_overall +smtp_confirmation
-smtp_incomplete_transaction +subject +smtp_connection

To debug mail delivery I examined mainlog. Problem is that I cannot
see the originating host. There are no log entries containing "<=":

Where is the problem? How can I fix that?

Thank you for your help.


Greetings,
--
R. Cirksena <ci@???>