Email warmup explained - why you can't just start sending

The most common cold email mistake is also the easiest to make: setting up a new domain and email account, then starting to send immediately. Within a few days, everything goes to spam. Open rates collapse. Reply rates are zero. The conclusion is that cold email doesn't work.

It doesn't work yet. The problem isn't the emails - it's that the sending infrastructure has no reputation. Email providers don't know whether to trust it.

Why email reputation matters

When an email arrives at Gmail, Outlook, or any other inbox provider, a series of checks happen before it's delivered. Some are technical (is the domain configured correctly?), some are behavioural (does this sender have a history of getting their emails opened, replied to, and not marked as spam?).

A brand new sending domain has no behavioural history. From the email provider's perspective, that's a risk signal. The majority of spam comes from new domains, because spammers constantly register new ones to stay ahead of blocks. So new domains start at a disadvantage.

Warmup is the process of building a positive sending history before you start your actual outreach campaigns.

How warmup works

Email warmup involves sending small volumes of email from your new account and having those emails receive engagement - opens, replies, and explicit "not spam" flags. The sending volume increases gradually over a period of weeks, training the inbox providers that this is a legitimate, engaged sender.

Warmup tools (Instantly has this built in, as do Lemwarm and similar services) automate this by connecting your account to a network of other accounts that send each other emails and engage with them. It looks like organic engagement to inbox providers because the signals are genuine - emails being opened and replied to - even though it's coordinated.

A typical warmup schedule looks like this:

• Week 1-2: 5-10 emails per day, high engagement rate
• Week 3-4: 20-30 emails per day
• Week 5-6: 40-50 emails per day
• Week 7+: Ready for campaigns at 50-80 emails per day

The total warmup period is typically four to six weeks before you're ready to send meaningful campaign volume. Rushing this is false economy - a warmed account delivers reliably for months; an unwarmed one gets blocked within days.

The technical setup: SPF, DKIM, and DMARC

Before warmup can work, the email authentication records need to be set up correctly. These are DNS records that prove you actually own the domain you're sending from.

SPF (Sender Policy Framework) is a DNS record that lists the mail servers authorised to send email from your domain. If your email comes from a server that's not on the list, it fails the SPF check. Setting it up takes five minutes in your domain's DNS settings.

DKIM (DomainKeys Identified Mail) adds a digital signature to outgoing emails. Inbox providers use this signature to verify that the email wasn't altered in transit and that it genuinely came from an authorised sender. Your email platform (Google Workspace, Outlook, Instantly) will provide the DKIM key - you add it as a DNS TXT record.

DMARC (Domain-based Message Authentication, Reporting, and Conformance) tells inbox providers what to do if an email fails SPF or DKIM checks. A basic DMARC record at p=none is enough to start; you can tighten it later once you're confident everything is configured correctly.

Without all three set up correctly, your deliverability will be poor regardless of warmup. Most email platforms flag missing authentication records - fix them before you do anything else.

Use subdomains, not your primary domain

Your primary domain (yourcompany.com) is the domain your website runs on, your primary email runs on, and your brand reputation is attached to. You don't want to risk it on cold outreach campaigns.

The standard practice is to set up a subdomain for outreach: outreach.yourcompany.com or hello.yourcompany.com. Warmup and campaign sending happens on the subdomain. If the subdomain develops a poor reputation or gets blocked, it doesn't affect your main domain's deliverability.

Register 2-4 subdomains and rotate sending across them. This distributes the sending volume across multiple domains and provides redundancy if one gets temporarily throttled.

Sending limits once you're warmed

Even with a fully warmed account, cold email sending has practical limits. Sending more than 50-80 emails per day from a single inbox pushes into risky territory for deliverability. With 4 warmed inboxes rotating, you can send 200-300 emails per day reliably.

For most outbound programmes at an agency or studio scale, that's enough volume. If you need more, add more domains and inboxes - this is intentionally designed to be modular.

The cost of skipping warmup

The most common outcome of starting to send without warming up: your domain gets flagged within 2-4 weeks. Open rates drop to 5-10% (most of the opens are from spam filters scanning the email, not humans). Reply rates are effectively zero. Eventually the domain is blacklisted and you start over.

Starting over means registering new domains, waiting another 4-6 weeks, and losing all the sending history you would have built if you'd done it correctly the first time. The 4-6 weeks you "saved" by skipping warmup costs you 8-12 weeks of lost campaign time.

It's the single most avoidable mistake in cold email. Do the warmup first.