The overwhelming consensus among experts, marketers, and email service documentation is that warming up a domain and/or IP address is crucial when migrating to a new ESP. This process involves gradually increasing sending volume to establish a positive sender reputation with ISPs, preventing emails from being flagged as spam. The main reason is that sender reputation does not transfer; moving to a new ESP essentially starts a clean slate. While some suggest that warming might be less critical for smaller lists when moving between shared IP ESPs, the advice generally leans toward a cautious, gradual approach to ensure deliverability. It's also important to monitor deliverability metrics and adjust sending strategies accordingly.