Outlook inbound deliverability
We're currently battling an attack on our infrastructure which is leveraging Microsoft email servers (specifically Outlook) to send up to 100k connections per minute our way.
This means that emails originating from Microsoft servers and trying to hit a redirection alias created on ImprovMX are soft-bouncing. Outbound traffic to outlook isn't affected, nor is any other email provider.
Unfortunately, there is no quick way for us to block the incoming traffic, but we've identified the cause of the problem and are working on multiple levels to get this resolved as soon as possible.
1) Human - if the ESP block the bad actor, the problem is solved in a minute. We've been very insistent, but if it doesn't work, we'll start legal action to get movement
2) At the Microsoft level. We've lodged complaints through several channels so that they stop sending all the bounce reports back for that specific IP range. Unfortunately, this is usually very slow and doesn't get solved quickly.
3) At our technical level - We've increased AWS instances to the maximum we can afford (we're 10xing costs for this month - erasing all profits basically). It's like increasing the size of a leaky bucket, but it works temporarily.