Can't send mail through public wireless network

[Alex]

That sums it up. Receiving mail is not a problem, but almost every
attempt to send through Outlook while on a public wireless network
fails, with the message from System Administrator stating
"Undeliverable: e-mail", with no further details. I suspect that the
ISP running the wireless network is not relaying any mail, but the
message says nothing about blocking relay. Is this the case, or is
there something else I'm missing? I can send mail fine by logging on
to my webmail.

Thanks,
Alex