Maybe this is because you are sending spam?

If your mails are not spam, then you will have to try to make them less similar to spam. Look at some spam mails and look what they have in common with your mails, and then change your mails.

<pFor example, mails that consist only of HTML and pictures, and no plain text part, are often considered as spam.

Another thing could be that you are sending mail from an IP address that is used to send spam.

You could check what SpamAssassin rules your mail triggers.

And in the end, you could contact the Yahoo! mail administrators.