What must I do so my e-mails don't get marked automaticly SPAM just because they are NOT sent from Yahoo srvrs

I send e-mails to my friend and clients to their yahoo e-mails from my own servers. I do this every week as a newsletter. I never spamed or send any kind of unsolicited e-mails, still, whatever I do or no matter how many of my friends/clinets put my e-mail in their Yahoo contact list and mark them NOT SPAM, they still end up in the SPAM or BULK inbox. I find this very frustrating and unfair.