Each domain/IP pair has an associated reputation score which is dynamically defined by the ISPs according to the email sending behavior. This reputation score will be unique at different ISPs based on their specific criteria. Email delivarabilty and inbox delivery will be directly proportional to this reputation score.
Also, to build up a stable and good reputation at various ISPs, at least 90 days of constant volume email delivery is required. So, no ISP will trust the sender if they’re seeing a number of emails from a source (IP/domain) for the first time and that will result in poor delivery rate. So, it’s very important that you should not start sending out emails at maximum capacity from day one.
We should have a step up procedure to increase the email volume day by day to the peak capacity. Suppose your daily sending volume is 12000 . So, what you have to do is, you can send 1000 emails on the first day. On the second day, you can send 2000 emails, 3rd day 4000, 5th day 8000 Just like that, you can double the limit day by day until your desired steady limit is reached. Whatever your desired daily limit is, warm up to that limit in 7 days.
This is in the case of using 1 SMTP. If you have two of them, start from 2000, step up the limit by 2x each day until you reach your desired limit. If you want to separately warm them up, you can do that as well.
As like stepping up/warming up the IPs, it’s important to keep a constant sending volume. It doesn’t mean that you should send the same volume of emails everyday, but there should not be a high level of zig-zag behavior in daily email volume. For example : it’s not advisable to send 15000 emails a day and then not sending for 7 days, again sending 2000 emails on the day after etc.