What is Email Warm-Up?
Definition: The process of gradually increasing sending volume on a new email account or domain by exchanging automated emails with a warm-up network, building sender reputation before launching cold outreach campaigns.
New email accounts have no reputation. If you start sending 200 cold emails on day one, Gmail and Outlook will flag you as spam immediately. Warm-up fixes this by simulating normal email activity: sending, receiving, opening, and replying to emails within a network of warm-up accounts.
The warm-up process takes 2-3 weeks. During this time, warm-up tools send 20-40 emails per day from your new inbox to other warm-up participants. Those participants open your emails, reply to them, and mark them as "not spam" if they land in junk folders. This signals to email providers that your account is legitimate.
Instantly includes warm-up as a core feature. You connect your inboxes, enable warm-up, and it runs in the background forever. Smartlead does the same. Standalone warm-up tools like Warmbox and Mailreach exist for teams using sequencing tools that don't include warm-up.
GTM Engineers typically maintain 5-10 sending inboxes per campaign. Each inbox warms up independently and handles 30-50 cold emails per day on top of its warm-up volume. The math: 10 inboxes at 40 emails/day = 400 cold emails daily, which is a solid volume for most campaigns without triggering spam filters.