E-mail is generally connected via either POP access or IMAP access and iPhone supports both of these standards. If an e-mail provider supports one of these methods they will document how to connect in the help section of their website.
Keep in mind that not all e-mail providers provide this access, but chances are good if you're able to connect using Outlook Express.
I don't have a RoadRunner account, but I believe you can login here ( http://webmail.roadrunner.com/) and then click on the Help section.