Can't Send E-Mail

I'm having trouble setting up my Roadrunner account through Thunderbird on my thumb drive. I can receive mail fine. When I try to send mail I get an error message:
Relaying denied. IP name lookup failed. Please verify that your e-mail address is correct in your mail preferences and try again.

I've double checked the smtp server. The status box shows connected to the smtp account when the error shows up. Anybody have an idea to fix this most frustrating problem?