We improved volunteers' inbox experience by displaying an error when they attempt to send a message to a contact who has already opted-out from receiving further messages. Previously, it was not clear enough for volunteers that this person could not be messaged anymore, and they would attempt to send a message in the brief time before the thread was auto-archived by Impactive (due to the opt-out); now, the error will make clear why the volunteer's message isn't going through to the contact.