It ALL depends upon what you're using yourself for a mail service.
e.g. if you have the correct reverse-DNS PTR (pointer) records in place for your mailserver, and whether they match the forward lookup. If you have SPF records in place. etc etc etc.
I'd hazard a guess you're sending mail out from a DSL originating IP range, to an SMTP server that itself is victim to being marked as a spam sender by the RBLs on a regular basis, and you're sending out mail content that possibly appends dodgy URLs in the signature, or has crap like Incredimail in the body.
How close am I?