Linux Email Server Setup

I need a Linux email expert to set up and teach me how to set up an email server to be used by multiple machines. Here's our setup: We have a front end machine running Ubuntu where we host our online VoIP service website. We have our SSL certificates installed there. Emails sent from that machine from our domain generally arrive with no problem (they are not marked as spam). We have our VoIP servers that also will send out emails. We can send a mail using our domain, but they almost always go to spam. We need to configure our VoIP servers to either route through the front end machine (with the certificate) or set up the certificate on the other servers so that we can reliably deliver email. At least, this is what I think we need to do. Read on! I'm sure this is an easy problem to solve, but I've never done this before. HERE'S THE CATCH: Your job will be to implement this on the front end server and one of the VoIP servers. We will do it TOGETHER with a desktop share through either Skype or Teamviewer. I'll drive and you'll direct. You are also to make your own suggestions on how to solve this problem. Please only accept this job if you are comfortable in working this way. Think of this as pair programming or rather "pair system administration". My timezone is Hong Kong, which is 12 hours off of USA Eastern Time. I'm fairly flexible in my time: 8am-11pm. I'll adapt to your schedule as best I can. I look forward to working together to get this done and to learn something new!