I need IP addresses for reputation

Number one of seven in our occasional series on why ESPs need, or don’t need, lots of IP addresses to send mail properly.

I need at least one IP address per customer, to handle IP based reputation

Why this is right
While DKIM is gradually moving the main key for reputation tracking to a domain based token, right now the main key that is used to track reputation is the sending IP address.
If you have multiple customers sending mail of different quality using a different sending IP address for each of those customers means that the good customers will not be penalized by the poor behaviour of the bad customers. And, just as importantly, poor customers will not benefit from the behaviour of the good customers. This allows receivers to track sender reputation more accurately, and so delivery just wanted email to their recipients better. That makes everyone happy (other than the bad customers who deserve to be unhappy until they fix their practices).
Why this is wrong
Reputation is tied to sending IP address, but it’s also affected by volume of emails received from that IP address, and the consistency of volume. If a customer is only sending a few hundred emails a week to any given receiver ISP or they’re only mailing monthly then they won’t be able to maintain much of a positive reputation, simply because they’re too small to keep track of or because they mail so infrequently that each time they mail the receiving ISP will have forgotten about their previous mailings. In those cases the sender will be treated much the same as a new sender from a given IP address (neutral, at best, maybe poorly). For those cases a customer is likely to get better delivery rates if their mail is sent through an IP address pool that sends enough email overall to be noticed and tracked by receiving ISPs.
Another reason this is wrong
Reputation is tied to sending IP address, but receiving ISPs aren’t stupid and do recognize attempts to game the system. If you’re an ESP with a mix of good and bad customers then segregating the IP addresses they send from will not completely isolate the reputation of those customers from each other. The bad customers will drag your reputation as an ESP down more than the good customers will pull it up. And as your reputation as an ESP degrades it will pull down the reputation of your good customers much more than it will increase the reputation of your bad or unknown customers.
So segregating senders onto their own IP addresses doesn’t entirely separate their reputation from each other or from their ESP. And if you believe it does, you’re likely to make business decisions based on that misunderstanding that will badly affect your reputation and the delivery rates of your customers. Don’t fall into that trap.