Spam filters work in many different and mysterious ways. Also you have several levels of filtering, your own ISP, and then your email client, for a start.
A couple of things I've realised is that the following are likely to be treated as spam.
- multi addressed emails where many recipients are in copy, specially if .bcc
- emails that contain the word Nigeria
- emails that contain large numbers preceded or followed by a currency symbol, such as $45,000,000
- obvious words such as Viagra, Cialis