Warming up a domain with a small email list involves gradually building a positive sending reputation. Experts recommend starting by sending emails to the most engaged subscribers with relevant, high-quality content. It's crucial to increase the volume and frequency of sends slowly, while closely monitoring engagement metrics like open rates, click-through rates, bounce rates, and spam complaints. Segmenting the list by engagement level helps prioritize active subscribers. Avoiding sudden spikes in email volume is essential to prevent triggering spam filters. Consistent sending habits, proper email authentication (SPF, DKIM, DMARC), and easy unsubscribe options are also important. Some also highlight tools like Google Postmaster Tools and Microsoft SNDS for monitoring sender reputation.