A new sending domain has no reputation. Not a bad one. None at all.
Mailbox providers treat an unknown domain that suddenly sends four thousand cold emails about the way a bank treats a brand-new account wiring $40,000 overseas. They assume the worst and act on it. Warming is how you build that reputation on purpose, before volume starts to matter.
Thirty days is the figure everyone quotes. It is worth saying plainly that no mailbox provider publishes one: it is a convention that works, not a threshold anything actually checks. The schedule below is a sensible default, not a specification.
What domain warming actually is
Domain warming is a scheduled ramp of sending volume from a new domain, paired with recipients who are likely to engage, so inbox providers accumulate positive signals before they see meaningful volume. Opens, replies, no complaints.
Three things are being built at the same time: the domain's reputation, the sending IP's reputation, and your authentication track record.
The common mistake is treating warming as a waiting period. Thirty idle days build nothing. What builds reputation is engaged mail, sent consistently, at a volume that grows slowly.
Set authentication first
Warming an unauthenticated domain wastes the thirty days. Publish SPF, DKIM, and DMARC before the first send. A warmed domain that fails DMARC alignment still lands in spam, and you will have spent a month proving it.
_dmarc.yourdomain.com TXT "v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com"
send.yourdomain.com CNAME → your sending provider
click.yourdomain.com CNAME → your click-tracking host
Start at p=none so you collect reports without blocking your own mail, then tighten to quarantine once the reports come back clean. In SEMAOS, DKIM and DMARC are configured out of the box. The two CNAMEs above are what you add on a paid plan to isolate your own sending and click-tracking reputation. The product tour shows the verification screen.
The 30-day ramp
Volumes below are per mailbox, per day, and they are a starting point rather than a standard. If you are sending from three mailboxes, triple them. Never triple by shortening the ramp.
| Week | Sends / mailbox / day | Send to | Watch |
|---|---|---|---|
| Week 1 | 10–20 | Contacts who already know you, replies expected | Any bounce at all |
| Week 2 | 25–40 | Warmest segment of the real list | Open rate holding |
| Week 3 | 50–80 | Broader list, still opted-in | Complaints, spam placement |
| Week 4 | 100–150 | Full cold segments | Per-provider divergence |
Week one is the part people skip, and it is the part that does the most work. Mail to people who already know you gets opened and answered, and a reply is the strongest positive signal there is. You are not prospecting yet. You are building a record.
Signals that mean stop
Pause the ramp and fix the list the moment any of these show up. They do not resolve on their own, and volume makes them worse.
- Hard bounce rate above 2%, the ceiling most senders work to. The list is stale or unverified.
- Complaint rate above 0.1%, which is the line in Google's sender guidelines. Targeting or copy is wrong, not the domain.
- Open rate falling week over week while volume climbs. You are being filtered.
- One provider diverging sharply from the others.
That last one is the most useful and the most ignored. Gmail and Microsoft make independent decisions. If Gmail placement holds while Outlook collapses, the problem is not your domain in general, it is something specific about how Microsoft reads your mail, and averaging the two hides it. Diagnosing a single provider is its own job.
Warming does not survive a gap. Two weeks of silence in the middle of a ramp puts you closer to the start than the end, so plan the thirty days around a period when someone is actually watching the numbers.
FAQ
How long does domain warming take?
About 30 days is the common convention rather than a published standard, and no mailbox provider states a number. Domains sending only to opted-in, highly engaged lists often reach full volume sooner. Domains that trip a filter early should expect longer.
Can I warm several mailboxes at once?
Yes, and it is the right way to add volume. Each mailbox follows the same per-mailbox ramp in parallel. Three warmed mailboxes at 100/day beat one mailbox pushed to 300/day, which looks like exactly the behavior filters are built to catch.
Do I need a separate domain for cold outreach?
A subdomain or separate domain keeps cold-outreach reputation away from your primary business mail. If a cold campaign goes badly, your invoices and support replies still deliver.
What happens if I skip warming?
Some mail lands, then placement degrades sharply, usually within the first few thousand sends. Recovering a burned domain takes longer than warming a fresh one would have.