We'd like to help! Just a couple of questions - are you using a @hughes.net email address? If so, are you trying to email from our webmail page or a separate e-mail client?
I have the same problem. I cannot respond or reply to an email sent to me unless i left click on the mouse and type while holding down the left mouse button. Only works with letters not correction keys (back space, delete). Use windows 10 and Hughes net software.
What browser are you using? Have you tried a different browser to see if the issue persists?