My spam filter keeps blocking anything in the subject line that starts with "RE:" as in when someone replys to a mail. I have this across all my domains.
The only SPAM filter that will be capable of this would be the bayesian filter (all the other filters work on things such as the IP or the SMTP recipient address). What you could do is adjust the bayesian threshold so it allows more mails through. Alternatively you could lower the threshold considerably, then in content filtering, create a rule which looks for a higher %, but ignores mails that have RE: in the subject.