It is because they block outgoing smtp unless you are on their network. When at home on Wifi, your WiFi is on their network. When out, you are on Edge. Many "cheaper" ISPs do this as a restriction as well as to protect their mail servers from being taken over for sending spam (though there are better ways).
Search here for CPMX.COM (or something like that) to see threads of how to use a free SMTP server to send when on Edge, but note, when at home, you won't be able to send while on Wifi (catch 22)
Message was edited by: DaVBMan