I don't believe flash would be at fault for this, Few things to Check.
1) Check Spamhaus to see if your servers IP/subnet address has been abused before.
2) Make sure you have reverse DNS setup for that server. A Good way to test this is to send to a Comcast email or an Aol address and see if you get a refusal message at your from address.
3) Spam filters generally work from a Score Value - Each thing it finds questionable will add to the score, after a certain threshold it will be sent to the Spam filter or rejected.
4) If you have this setup where the from email address is the Senders and the To address is the recipients - this might cause for alarm because not one address sent through this server is resident on this network via reverse DNS.
5) Have this system send you a message and Check the headers. I suspect you might find some additions to the headers that state that the email originated from Apache. Compare the headers against other emails from your outlook message program - See how close you can structure to those (without falsifying information which is illegal). a lot of the Scoring is done by the headers.
Just a few thoughts, perhaps someone else has something to add. Email is a very abused mode of communication, as so the rules change everyday and it's a nightmare to guarantee delivery anymore.