don't freak out about Reverse DNS, basically you have to send an e-mail to whoever your ISP is, or whoever owns your IP addresses you're using, just tell them to add something like:
reverse DNS entry of mail.yourzimbrabox.com for your.public.ip.address
and they'll know what to do. SPF is little more sticky, but basically once you get reverse DNS done, it'll probably deliver most e-mail until you figure out SPF, reverse DNS missing will hoze you though. You can check reverse dns to see if it's right by typing
dig -x your.public.ip.address and see if it says mail.yourzimbrabox.com |