RoadRunner mail problem: receive but not send

I have a new Mac desktop (two weeks old). Using the mail program that came with the system...it set up itself upon starting the Mac...I can receive email but it will not send email. My server is smtp-server.san.rr.com and TimeWarner/RoadRunner tells me that is correct. Anyone know what the problem is and how to solved it?

Go into System Preferences > Network > Airport > Advanced > DNS and ensure there are three IP addresses appearing under DNS servers. The first should be greyed out, your machine IP and then a Primary and Secondary address or your ISP. If they do not appear, contact your ISP and ask for them.

Next step is to check with the ISP what you have entered in outgoing mail in Mail preferences is correct. e.h.. mine just shows the ISP name without the actual address.