How to warm up a cold email domain

A brand-new sending domain has no reputation, so inbox providers treat it with suspicion — send real volume on day one and you land in spam. Warming is how you build reputation gradually so your mail reaches the inbox.

Here’s the process, and the one thing that quietly undoes all of it if you skip it.

Before you warm: get the setup right

Register a dedicated sending domain (not your primary), and publish SPF, DKIM and DMARC on it. Warming an unauthenticated domain is wasted effort — the mail is suspect no matter how gently you ramp.

Set up a mailbox with a real name and signature. Reputation attaches to a domain that looks like a person, not a faceless sender.

The ramp: slow and steady

Start with a small number of sends per day to engaged, real recipients, and increase gradually over several weeks. The exact curve matters less than the principle: never spike. A sudden jump in volume from a young domain is the classic spam signal.

Warm-up tools automate part of this by exchanging positive-engagement mail, but the reputation you build is only as good as the real lists you eventually send to.

Engagement is the real currency

Opens, replies and "not spam" signals from real people tell inbox providers your mail is wanted. Prioritise recipients likely to engage early in the warm-up, and keep your content genuinely relevant.

Conversely, spam-folder placement and deletions-without-open drag the reputation down. Warming is really about accumulating positive signals faster than negative ones.

The thing that undoes warming: bounces

You can warm perfectly for weeks and then torch the domain with one dirty list. A batch of invalid addresses spikes your bounce rate, and inbox providers immediately re-rate you as careless. Above 5% bounce is dangerous territory.

So the day you start real sends, verify every address first — and make sure your verifier doesn’t just SMTP-guess on Microsoft 365, because those guesses are the hidden bounces that undo a warmed domain. Protecting the reputation you built is exactly why verification matters.

Takeaway

Authenticate, ramp volume slowly, chase real engagement — then protect it all by verifying every list before you send. Warming builds reputation; a dirty list (especially unverified Microsoft 365 addresses) destroys it in a day.