[exim] String expansion sometimes not working

I am trying to setup per-user configurations for spam scanning. In my
acl_smtp_data checks I have the following:

*** START ***
accept condition = ${lookup mysql{SELECT spamscan FROM users WHERE
email='${quote_mysql:$local_part}@${quote_mysql:$domain}' AND
spamscan=0}{yes}{no}}
*** END ***

This query sometimes works correctly, but mostly fails to pass the
recipient information to the DB:

*** START ***
13141 Query       SELECT spamscan FROM users WHERE email='@' AND spamscan=0
13148 Query       SELECT spamscan FROM users WHERE
email='iain@???' AND spamscan=0
13149 Query       SELECT spamscan FROM users WHERE email='@' AND spamscan=0
*** END ***

The query is taking place once the data part of the SMTP process is
being run, so I am at a loss as to why Exim does not pass this
information in a consistent manner.

Any ideas?

Iain.