block messages having no subject using acl in exim

add these lines in your exim.conf

acl_check_data:
deny condition = ${if or
{{!def:h_Date:}{!def:h_Subject:}{!def:h_To:}}{yes}{no}}
message = Message does not conform to RFC2822 standard

deny condition = ${if eq{$h_Subject:}{}{yes}{no}}
message = Messages with blank/empty subjuect line are not accepted