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What you will learn

How to build sending reputation with a list that has never received your email (or hasn’t in a long time) — slicing, measuring and growing with each send.

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Why warm up

Mailbox providers judge senders by recent behavior: sudden volume from a new sender, with bounces and complaints together, is exactly the signature of spam. Thousands of cold emails at once is the biggest reputational risk a new account can take — and a burned reputation takes weeks to recover. Warming up trades one big risk for several small, measurable ones.

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The slice plan

  1. Start with your most engaged people. First slice of about a thousand contacts — the ones who bought or opened something recently. If your list came from another tool, use its history to choose.
  2. Measure before you grow. After the send, open the Delivery of sent section of the campaign report: hard bounces below 2% and complaints below 0.1% are the limits — above them, the row is tagged above threshold. Within them, double the slice on the next send; outside them, stop and investigate before continuing — the problem is usually list quality, not content.
  3. Repeat until you cover the list. Doubling after each healthy send reaches your entire list in a few weeks, with your reputation built along the way.

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Use send pacing

Even within a slice, spread it out: when setting up the campaign, in the Delivery step, Send pacing distributes the send over 1, 4, 8 or 24 hours instead of releasing everything in the same minute (Instant). For large lists, this avoids the spikes that trigger blocking at the providers. Pacing is self-correcting — the campaign finishes at the scheduled time.

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What to watch on each send

  • Bounces — above 2%, the list has dead addresses: clean it before growing.
  • Complaints (spam) — above 0.1% is a red flag: review consent and frequency, not just the subject line.
  • Unsubscribes — healthy is below 1%; a spike means the wrong audience for the message.
  • Needs attention — the bell warns you automatically when the bounce rate goes above 2% or the complaint rate goes above 0.1%, measured over the sends of the last 30 days (from 20 sends up). Unsubscribes don’t show in the bell: track them in the campaign report.

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What about WhatsApp?

WhatsApp has its own version of this: sends in waves, with the number’s daily limit and automatic safeguards that pause the campaign if blocks rise or the number’s quality rating drops. It’s covered in How to send a bulk message.