Viewing the Mass Mailing > Send a Message > Draft screen, I get the screen you see in this attachment:
I noticed this the other day, but was in a hurry to click to a different screen. Today I have some time so even though the test message was sent and received more than 10 minutes ago the screen has not refreshed. I have just now clicked the “Thanks!” button and the middle box disappeared, but the bottom box is still there.
This might be unnerving to a new user who might freak out wondering if their message was in a queue or had actually been sent.