Apple Mail 10.3. I just downloaded Sierra 10.12.5 on my MacBook. I had to set up mail and it automatically started downloading a lot of email that I had already read. I opened the screen all the way blocking the body of the message so that all I could see was the email senders so I could easily delete all of the unwanted emails. So now my issue is for some reason I cannot move that curser back up to see the body of messages. It simply will not move.
This can sometimes be cured by removing mail accounts first and then adding all your mail accounts again. It won't affect your mail accounts in any way even if you remove it since the mail is on the server and mail will keep any messages downloaded permanently from the account.
You could make a backup if you are worried about losing mail.
Also - be sure a simple reboot doesn't unjam things.