If you are having trouble with your emails being detected as spam, try running one or two of them through something like SpamAssassin, and seeing what tests detect spam-nature. (The tests are usually named pretty sanely, but go here if you get stuck.) Once you know what to avoid, you'll have better luck avoiding it.

For what it's worth, I don't think there's anything about the content of your message that makes it spammy, except for perhaps the shortness of the body. That might mean the tests you're failing are connected with your IP, hostname, domain, etc. rather than with your message itself.