Why My Domain Is listed as Spam

i have registred a domain a while ago and i’m running Qmail Linux mail server as an e-mail server.
When i send an e-mail to Yahoo, Gmail , Hotmail and other accounts, My e-mails are being listed as spam ….
How can i take my domain off the spam list?
I have to mention that i don’t have a static IP address assigned to my line. It’s only a Dynamic IP i get from my ISP used with DDNS …
i will appreciate any help on this.