Unable to reply to emails in Mail 9.3

I am using a MacPro, Retina, 13-inch, Early 2015 running OSX El Capitan v 10.11.4.

In Mail 9.3 I receive all messages normally but when I select 'Reply' or 'Reply All' I am presented with a small template that is devoid of any detail (no names, previous message etc.) - just a blank new message template. I cannot do anything with that template - I can't write in it, I can't delete it - nothing. All I can do is minimise it to the dock. Then I have to close Mail completely - re-open - and, with luck, it has cleared. The strange thing is that occasionally it all works fine.

Other forums have suggested deleting 'user > library > preferences > com.apple.mail.plist' but this is not present in El Capitan.

Any ideas anyone??