Mailbox warmup
Also called: email warmup · warm-up
Mailbox warmup is the practice of ramping a new sending mailbox from a low, steady trickle of real engagement up to full cold-outbound volume over several weeks, so mailbox providers learn to trust the address before it sends at scale.
A brand-new mailbox that sends fifty cold emails on its first morning is asking to be flagged. Reputation gets built by behaving like a real person before behaving like a campaign. Warmup means starting with a trickle, a few sends a day of low-stakes mail that gets opened and replied to, then ramping the volume gradually over a couple of weeks.
The exact ramp schedule matters less than the principle underneath it: slow, consistent, and boring. A mailbox provider learns from that pattern that the address sends mail people actually want. Rush it, and the provider learns the opposite lesson, which it remembers far longer than the time saved by skipping warmup in the first place.
Warmup is a stage, not a fixed number of days
There's no honest single number for how long warmup takes, and any post that gives one is guessing. It runs in stages: volume increases in steps, and each step only happens once the signals from the last one look healthy, bounce rate low, complaint rate near zero, opens holding steady. If those signals wobble, the right move is to slow down or hold, regardless of what week it technically is.
Warmup used to be a separate purchase. Increasingly it isn't. Sequencers including SmartLead now ship warmup natively, which is worth checking before buying a standalone tool like Mailreach or Warmy to sit alongside a sequencer that already does the job.
In practice
The count of mailboxes matters less than the blast radius: if one mailbox gets flagged or lands in a spam trap, how much sending capacity gets lost while the team sorts it out. More mailboxes spread that risk thinner, but each one needs its own warmup and its own health checks, which is the real reason teams add more mailboxes than the volume math alone suggests.
What people get wrong
Warmup gets treated as a one-time setup step to clear before the real campaign starts, then forgotten. It isn't. Every new domain or mailbox added later starts back at zero and needs the same ramp, no matter how well the rest of the sending infrastructure is running. Skipping it on a new domain doesn't just risk a slow start. It produces the exact burst-volume pattern spam filters are built to catch, and the reputation that would have taken a few weeks to build gets damaged before a single email that mattered goes out.
