This depends on who you're using to send your messages. If you're using your ISP's SMTP server, you may only be allowed to connect to it via a connection to that ISP, as you do at home. You need an authenticating SMTP server, which takes a login with a username and password to confirm that you're a bone fide user. All the big free email services already do this: gmail, yahoo,gmax, hotmail/live mail/outlook.com. It tends to be ISPs that drag their feet and offer only the bare minimum. A further consideration is that some providers block "commonly-used" ports, so if your SMTP server works via port 25, it is unfortunately for you likely to be blocked by many other ISPs or hotspot providers. 587 is a commonly used alternative to 25, and 465 is used with encrypted connections. Check if your ISP offers an email configuration aimed at mobile users. So, who provides your email service?