I've been having endless problems sending email to yahoo. I basically concluded that they have black listed my IP because when I was testing Zimbra I did not have a proper PTR record setup. So, now my IP is in Yahoo hell and there is not way to get it out (I think). I've filled out their spam form about 10 times and nothing changes. I don't want to mess with changing my IP (for now).
So, I did some research on how to setup a DKIM relay for my Zimbra box. I came across a great script called iRedmail - Installation - iredmail - Installation Wizard. - Google Code - it's a ready to go relay, mxbackup, email server - based on postfix and amavisd (with DKIM installed).
So, I downloaded the ISO and installed a VM on my ESX server. I'm testing it now, but I'm able to relay Zimbra email through iRedMAil and DKIM sign emails just fine. I ran into a couple snags that I talk about in the forum over there, if anyone is interested. This is the first postfix email server that I've come across that has a great configuration script that takes care of ALL the configuration...