Sep. 12, 2019 - This issues has been resolved and delivery rates are back to normal.
Sep. 11, 2019 - Starting early Wednesday morning we started experiencing email delivery failures when sending to Yahoo email addresses. We are working on moving to an IP with better reputation and should have this resolved later today.
Seeing an Issue with messages to Microsoft services intermittently bouncing with "554 Transaction failed". In reviewing, this looks to be an issue that is occurring on Microsoft's end and they are investigating. We will continue monitoring the situation and provide updates. Seems to have been fixed on Microsoft's end.