I support wireless data cards for a living and a very common issue that I see is that people will use their cable providers smtp server. In a lot of cases they don’t allow for relaying if you look in your mail program on your desktops, the ones connected to you cable modem, see if the outgoing server requires authentication, I doubt it does, in which case you would need to use a different smtp server or ask your provider if they offer a solution.
If it does require authentication and the checkbox is check and its set to use incoming settings and your still getting errors on the smtp with the data card double check your server name,. port number, username, and password. It is not common to have separate usernames and passwords for the smtp but its not impossible so check with the cable provider although it is very unlikely. If all checks out paste back the error message from the mail program your using so it can be troubleshot. If all else fails you could use a different smtp and use an alias to show a different return email address.