Due to a backend service going down on 11/3/2015, the Marketing Campaigns frontend went down at 7:02am MT. Mail flow in both mail processing data centers was delayed beginning at 7:40am MT, and mail for Marketing Campaigns were effectively paused starting at 8:30am MT. By 8:31am MT, the Marketing Campaigns frontend was back up, Marketing Campaigns mail started sending again and transactional mail started processing again.
New transactional mail was no longer delayed starting at 10:28am for all customers, but 5 million messages took until 8am MT on 11/4/2015 to finish sending due to a spike of traffic in our West Coast data center.
SendGrid is working on rebalancing our capacity to help prevent future incidents like this.