Webmail is an email client via the web and accessible with any browser, anywhere there's internet connectivity.
Using Webmail doesn't preventing a user from using other methods to access the email store, but depending on the connection and location, other email clients may have trouble sending email.
Using an ISP that is unknown to the email server results in a "relaying not allowed" error for sending email.
Known ISPs are SaskTel and Access.
Because the Webmail server is known by the Email server, such errors never occur even though the user may be in Australia.