Delays for Newsletters Sent to Microsoft Inboxes

By delays are you talking about deferrals?

The way we monitor these for our clients is we will look after a large volume send and see if the deferral rate is spiking. If it is, that is often a sign of too much volume in a given period. Microsoft is fairly temperamental with deferrals so there may be a need to divide out the Microsoft domains into a slower sending path with a canvas or duplicate campaign for a period. Building up the rate limit at which you send to Microsoft specifically.