I've followed the howto guides here and I'm very pleased they are here. I just wanted to point out a website that allows for some very nice testing. It even test smtp connections to the IP given. Also, it's a great way to see if your an open relay for spammers.
Also if your on a static IP it will let you know if your records are all ok. If not that will help you troubleshoot your connection, or correct your records.
Lastly, thank you Falko for your contributions here for Ubuntu if it weren't for them I wouldn't have a clue what I was doing. A tip for others following guides, just don't blindly copy and paste. Look up the commands used and become familiar with them, it will only help for the long run if you do.