Keeping it simple, and without any actual text....
...a black, right-pointing, arrow (intuitively meaning "next")...
...... and a grey check mark (intuitively meaning "done")...
Although.. I'd also maybe want to alter the text at the top and screen that back as well....
As for "other areas" outlined in the question.. I'd change the "message read" from √√ to simply √. If an "unread" icon is needed, then an empty square or circle.