How can I send emails via someone else's wi fi

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?