Why it is necessary
A brand new mailbox has no history with any receiver. When it suddenly sends four hundred messages on its first day, that pattern matches spam behaviour more closely than it matches a person, and receivers respond accordingly. Warmup establishes a gradual, human-looking sending pattern with positive engagement before real volume arrives.
What a schedule looks like
Start at a handful of sends per day per mailbox and increase gradually over two to four weeks, ending somewhere around thirty to forty per day for cold outreach. The precise numbers matter less than the shape: a smooth ramp with no spikes, and consistency including weekends.
Warmup tools work by exchanging mail between participating mailboxes and generating opens and replies. Replies are the valuable part — reply rate is a much stronger positive signal than opens, particularly now that open data is unreliable.
The mistakes
The two big ones are launching before warmup finished, and stopping warmup entirely once campaigns begin. Most operators keep a reduced level of warmup running permanently, because it maintains positive engagement as a baseline underneath the cold traffic, which by nature engages far less.
The third is treating the warmup ceiling as a suggestion. If a pool warmed to thirty a day and you push it to eighty because a launch needs volume, you have undone the warmup — the fleet needs to be sized for the target rather than stretched to reach it.
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Common questions
How long does warmup take?
Two to four weeks for a new mailbox on a new domain. There is no shortcut — the timeline is set by how fast receivers are willing to update their view of you.
Should warmup keep running during campaigns?
Usually yes, at reduced volume. It maintains a floor of positive engagement beneath cold traffic that by definition engages far less.
Can I warm up faster with more warmup emails?
No, and trying makes it worse. Volume ramping too quickly is the exact pattern warmup exists to avoid. The constraint is time, not send count.
Related concepts
Everything above is diagnosis. To confirm a fix worked you need placement data — seed accounts across the major providers, plus the DMARC aggregate reports read as a trend rather than a wall of XML. GlockApps is what I run for both.
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The pages explain it. If you would rather it was simply fixed, that is the work I do.