mail got stuck at this view... I can't see anything below this. I cannot even quite.

I think you mean quit. The email address you put in the To: field is an invalid email address.

Hit the Escape key in the upper right hand corner to see if the popup window disappears.

If that doesn't work, hit FN-Pg Down, or command down arrow. If nothing moves, you may have to force quit the email application, losing any changes you made since you last started composing the message.

To force quit use command-option-escape and select the Mail program and force quit.

When you relaunch Mail, hold the shift key. It should bring back the Mail application fresh

Make doubly certain the email address is of the format

user@host.suffix

If there are a mix of hyphens and numbers all over the place, locate the sender's website and tell them the email you received from them either is a phishing email, or was sent from an address you can't reply to.

Make certain you were expecting an email from them, before assuming you should be able to reply to the message.