To stop recieving mail or spam with blank from fields in exim use the following acl:

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.