Buffer / pre-warming
Sending a small, gradually-increasing volume from a new IP or domain to build sender reputation before scaling.
Buffer or pre-warming describes the practice of slowly ramping send volume from a new IP address or domain over several weeks to build a positive sender reputation with mailbox providers. A cold IP that suddenly sends 100,000 messages will be flagged as suspicious; the same IP sending 100 on day one, 200 on day two, scaling to 100,000 over three weeks, builds gradual trust.
The mechanics: mailbox providers (Gmail, Outlook, Yahoo) maintain reputation scores per sending IP and per sending domain. New senders start with no reputation. Warm-up signals are engagement (opens, clicks, replies) and absence of negative signals (spam complaints, hard bounces). The slower you scale, the more positive engagement accumulates relative to volume.
Modern ESPs like SendGrid, Mailgun, and Postmark provide automated warm-up tools. Cold-outreach platforms like Smartlead and Lemwarm rotate emails between participating inboxes to simulate engagement. Whether automated warm-up actually moves the needle vs. simply waiting is debated; what's not debated is that skipping the ramp entirely tanks deliverability.