Error when sending an email to class attendees

Tried to email all of the attendees for Basics of WordPress today and got this error: http://s21.postimg.org/4j5z75awn/Class_Mate_Email_Error.png

Not sure if the email sent or not, but I had checked the box to email instructors as well, so we'll see if I receive anything.

Not sure if anyone is watching this project in GH, so I'm emailing Lark directly as well.
