I was having the issue that if clamav died (usually due to freshclam update taking too long) that exim4 would start temporary rejecting ALL mail.

Here's a suggestion from Mike Cardwell on the exim users mailing list. It adds a check on the file existing and adds a header if not instead of rejecting.

warn !condition = ${if exists{/var/run/clamav/clamd.ctl}}
     add_header = X-Virus-Checked: False
deny condition  = ${if exists{/var/run/clamav/clamd.ctl}}
     malware    = *

See also http://lists.exim.org/lurker/message/20070918.172526.ff9818ec.en.html for a more detailed method.