Domain warming is the gradual increase of the volume of mail sent from a new domain or a new tool, so that it builds up a reputation with mail providers. Providers judge a sender by its history, and a domain with no history that suddenly sends ten thousand messages looks exactly like a spam sender. The process is slow and tedious: start with a small volume of messages to your most active contacts, who are most likely to open them, and increase the volume gradually over several weeks, while monitoring deliveries, opens, and complaints. If the metrics worsen, reduce the volume and wait. It is worth planning warming in advance when changing tools, when moving to a new domain, and after a longer break in sending. When switching between tools, don't forget to also add the domain's authentication records, otherwise reputation is being built for nothing.
See also: Email Deliverability, SPF, DKIM and DMARC, Unsubscribe and complaint rate.
