Email warm-up: raising send volume slowly

How it works

Mailbox providers judge mail partly by its history. A domain with no history, or one whose volume suddenly jumps, gets treated with caution. Warm-up builds that history on purpose: small sends to the people likeliest to open, then larger ones, checking bounces and complaints between steps.

Google's sender guidelines say to start with a low volume to engaged users, increase it slowly, and avoid bursts. Suddenly doubling your usual volume can bring rate limiting or a reputation drop. Resend's warm-up guide applies the same idea whenever you use a new domain, IP or provider, or your volume will grow. It asks you to keep bounces below 4% and spam complaints below 0.08% while you ramp up, and to slow down if either rises.

A typical plan for an existing list moving to a new provider:

1. First send: a small cohort of your most engaged subscribers.
2. Next send: that cohort plus another engaged batch, if the first results were clean.
3. Then the rest, once bounces and complaints stay low.

Why it matters

• The first big send is the riskiest. Skip warm-up and it can be throttled, deferred or filtered to spam.
• Damage outlasts the send. A reputation drop affects the sends that follow, not just the one that caused it.
• Switching platforms counts. A new sending provider, and often a new domain, starts with little history.

How Mimeo handles it

Mimeo doesn't deliver your mail: your own Postmark or Resend account does. Warm-up is a plan you run, and Mimeo gives you the controls for it. Its migration docs set out a three-newsletter transition: a small, engaged first cohort, expanded only when delivery results hold.

You pick each cohort with a segment, on a migration tag such as migration-wave-1 or on engagement data you brought from the old tool. Every broadcast drains at a set pace, most engaged people first, so the warmest addresses lead and a send going wrong can be canceled with most of it still queued. After each send, reports show bounces and complaints. See Mimeo terms for warm-up in Mimeo's own words.