It appears from the message header you posted that you sent the message to a Gmail account. I assume that the message was delivered to the SPAM or JUNK folder in the Gmail account.
I had the same issues until I installed dkimproxy. Gmail uses DKIM signatures and Yahoo uses DomainKeys signatures.
Once I started signing outgoing email messages, mail has a better (although not perfect) record of being delivered to the Inbox rather than a Junk folder.
My base system is a Debian 5 (Lenny) server with ISPConfig3 setup using the How To on this site. For the dkimproxy setup, I had to look to other tutorials available on the Internet. It was a lot of trial and error. Lenny does have dkimproxy in the repositories which can be installed with 'apt-get install dkimproxy', but I had problems getting it to work correctly. I eventually built a version from source using another tutorial.