I've seen sometimes (can't remember why) that connecting to localhost doesn't work, but from another IP or on the public address it does (or was it the other way around?).
Have you tried without firewalling?
Re hostname: doesn't matter. What matters is that the domain you've set your server as MX for is listed as destination in Postfix. And of course, the host you list as MX should have an IP-adress in DNS listed.
Oh, and BTW, check your SQL-dumps for passwords if you put them online. :-)