For a large retail company migrating to Marketing Cloud, proper IP warming is critical:

A. IP ramp-up takes four to six weeks to be able to fully send to all 3 million customers: It's essential to gradually increase the volume of emails sent from a new IP address to build a positive reputation with Internet Service Providers (ISPs).

D. IP ramp-up is important to establish a positive sender reputation: Establishing a positive sender reputation with ISPs during the IP warming process helps ensure high deliverability and avoids being flagged as spam.

These responses highlight the necessity and methodology of IP warming when transitioning to a new email platform to maintain deliverability standards.