Why a new mailbox needs 21 days before it can carry a campaign
A fresh domain has no history, and mailbox providers treat "no history" as "unknown risk". Warmup is the process of manufacturing a history worth trusting.
7 min read · Written by the Autocloz team
What a reputation actually is
Gmail, Outlook and every other major provider score the domain and IP a message comes from. That score is built from observed behaviour: how many messages you send, how steadily, how many bounce, how many recipients open, reply, archive or mark as spam.
A brand-new domain has none of these signals. It is not trusted and it is not distrusted — it is unknown, and unknown traffic gets filtered conservatively. Sending 500 cold emails on day one is the single fastest way to convert "unknown" into "spam".
The three things warmup is building
- Volume tolerance. Providers learn what normal looks like for you. A domain that has never sent more than 20 messages a day looks compromised the moment it sends 400.
- Positive engagement. Opens help, replies help more, and a message moved out of spam into the inbox helps most of all. Warmup traffic is designed to generate exactly these.
- Authentication history. SPF, DKIM and DMARC records need time to be observed as consistently passing, not just present.
Warmup is not a workaround for bad targeting. It earns you the right to be delivered; it does not make a bad list into a good one.
Why 21 days
Twenty-one days is not a magic number, it is the practical floor. Reputation systems average behaviour over rolling windows, and most need two to three weeks of consistent signal before a domain moves out of the provisional band.
A typical ramp starts at 5-10 messages a day in week one, 20-40 in week two, and 60-100 in week three, with volume increasing only while bounce and complaint rates stay low. If either metric rises, the ramp pauses rather than continues.
Domains that reach full volume faster usually do so because they were bought aged, or because they inherited reputation from an established parent domain. Neither is available to a brand-new subdomain.
What breaks a warmup
- Jumping volume. A step change from 40 to 400 undoes weeks of ramp in a day.
- Sending to a stale list. Bounces above roughly 3% signal that you are not verifying recipients.
- Pausing entirely. Reputation decays. A mailbox that sends nothing for three weeks partly resets.
- Using your primary domain. If outbound burns the domain, it takes your invoices and password resets with it. Send from a separate domain.
How Autocloz handles it
Autocloz runs warmup on your own mailbox rather than a shared pool, so the reputation being built is genuinely yours and follows you if you leave. The ramp is paced automatically and holds volume steady whenever bounce or complaint rates cross the threshold, rather than continuing on a fixed schedule.
Campaign sending from a mailbox still inside its warmup window is capped to the ramp, so a new user cannot accidentally launch 500 sends on day two.
Common questions
Can I skip warmup if I use a well-known provider like Google Workspace?
No. Reputation is scored per domain and per sending IP, not per provider. A new domain on Google Workspace still starts with no history.
How many mailboxes do I need?
Divide your target daily volume by roughly 50 sends per mailbox per day. Sending 200 a day sustainably means about four warmed mailboxes, not one mailbox sending 200.
Does warmup ever finish?
The ramp finishes, but reputation is continuously scored. Volume that drops to zero for weeks and then spikes will be treated as new traffic again.
Should I warm up my main company domain?
No. Use a separate domain for outbound so that a deliverability problem never touches your transactional and internal email.
Warmup is included, on every plan
Autocloz paces your ramp automatically on your own mailbox. Free forever, no card.