Hmmm. In this case, I think it means more than one woman is closing her email program. Unlike many people who leave their programs open all the time and receive a notice of each new email, for security reasons I close my program any time I'm not actively using it. Maybe this sentence refers to me and other women like me. In any case, DL accepts "They are closing their email." I didn't try "They are closing down their email."
Okay. In at least 4 months no one at Duo has thought to pull or rewrite this sentence. While you can close a program, you can't do what this sentence says and close an email. Please change it Duo. How about 'ventanas'? Ellos cierran sus ventanas' You know ... because this sentence stinks they closed their lesson window. ?No? ?No pun jokes? Just kidding. I love this program! lol