The BlackBoard hosted Mail tool in our Vista course management system began to malfunction at some point over the weekend. The problem was not readily noticed by students and faculty or at least was not reported until Monday. The tool has been repaired but the back-up of the mail could take 24 hours to clear.
It may be helpful for you to post a discussion message or announcement in your Vista courses to let students know that if they have sent you an email to which you have not responded, you are not ignoring them! Encourage them to contact you via your external emergency email address if they have urgent issues. Thanks!