i had the same problem. roadrunner, right? i was using smtp-server.kc.rr.com i could send email when connected to t-mobiles network, as soon as i connected to wifi i could no longer send email. i could receive email just fine on either connection.
finally after going back and forth with road runner and t-mobile i finally got someone at t-mobile that had a solution
use mobile-smtp.roadrunner.com and server port 25. then you can send email over wifi or t-mobile connection
i had a thread on this on the old support forum but its gone now.