Email Warmup

Also known as: domain warmup, mailbox warmup

Email warmup is the process of gradually building sender reputation on a new email domain or mailbox before sending cold outbound at scale. The process involves sending small volumes of emails to recipients who reliably open, reply, and mark as 'not spam' — signaling to inbox providers (Gmail, Outlook) that the domain is legitimate. Warmup typically runs 4-6 weeks before any cold outbound starts.

Skipping warmup is the most common cause of cold-email infrastructure failure. A brand-new domain that sends 100 cold emails on day one will almost certainly land in spam, and once a domain develops a spam reputation it is extremely difficult to recover. The opportunity cost of skipping 4 weeks of warmup is months of degraded inbox placement.

Modern warmup uses pools of cooperating mailboxes that send to each other, open the emails, and reply — a synthetic but inbox-provider-acceptable pattern that builds reputation. The quality of warmup depends on the pool: a small pool with low-quality engagement signals can do as much harm as no warmup at all.