Can’t send email when I’m away from home.

Are you using an email service provided by your internet provider? If so, the inability to send may be deliberately enforced by your internet provider. If you access the web portal for your email, you should be able to send/receive there when you could not while using the mail app.

Can you access the internet on your iPhone when you are away from home? Do you see an error message on the screen? If you are not able to access the internet I would say you do not have a data plan. Can you please provide more information?