This line :
X-YahooFilteredBulk 122.169.116.246
Means that your IP is in one of Yahoo blacklist.
You should try to fill this form :
Yahoo! Mail Delivery Issues Form | Yahoo! Postmaster Help
Also this one if you use mailing list in your organisation :
Yahoo! Mail Bulk Sender Form | Yahoo! Postmaster Help
Finally, Like it's been said before, DKIM could also help, see the following post on my blog on how to do that :
Installing DKIM for Zimbra on CentOS or Red Hat | Le blog d'Étienne Pouliot
Another solution could be to relay all your mail to your ISP SMTP.