Spam filters are a tricky beast. Some of the filters could have determined that your email or newsletter contained "spammy content". Spammers and spam filters are constantly playing a game of cat and mouse. Once spammers figure out a way to get into inboxes, spam filters react, leaving the legitimate senders stuck trying to figure it out
Here are two websites that provide spam "buzz words" to keep out of your subject line and message:
Also, this link will give your message a spam rating. Just send your message to the email address they provide and their software will test your content and a few other items and give your message a spam rating.