Not sure if I am understanding you correctly so I will provide two suggestions.
If you are trying to send an email from your server to your Yahoo account and it is getting tagged as spam you should send mail via your ISP's SMTP server. Find out if one is provided by your ISP and add it in Server Manager. Some larger companies like Yahoo may block mail that originates from a private address range.
If you have setup your server to forward all mail to your yahoo account then just add your Domain to your Yahoo whitelist. Or follow step 1.
hope this helps