Any deliberate maliciousness on their part aside, at the very least they could have "Warmed-up" the new IP addresses before switching customers over. That's like moving your existing customers over to a new server and only then having them wait while you "install software". At the worst, it's malicious. And at the best, just plain incompetent.
Again, should they not be able to do that before using the customers as guinea pigs? You'd think that they'd have a large buffer of "untainted" IP's that they slowly cycle towards. I.e. they "slowly" warm them up with legitimate emails of their own, and then have them on hand for when they need to replace existing IPs for customers.
The point I was trying to make is this: in the example you gave ("waiting to install software") or the one I gave (warming up a car on the driveway), you can get the system ready for action without creating any impact in a production environment.
The case of email servers is an unusual example where you have to do real world tests in order to make the system work. In most other situations, you can do everything before the rubber hits the road.
So, whilst this could have been handled much better, it's not a simple situation.