What Is Cold Email Infrastructure and Why Does It Matter?

Infrastructure is the foundation that determines whether your cold emails land in inboxes or spam folders. It is also where 80% of agencies and in-house teams cut corners.

If you have ever launched a cold email campaign and watched your reply rates sit at zero, the problem probably was not your copy. It was not your offer. It was not your targeting. It was your infrastructure.

Before a single prospect ever reads your subject line, a series of technical systems have already decided whether that email reaches them at all. Those systems -- your domains, inboxes, authentication records, and warmup processes -- are your cold email infrastructure. And if any one of them is poorly configured, the rest of your campaign effort is wasted.

So what does good infrastructure actually look like? What does it cost? And why do so many businesses get it wrong?

What Cold Email Infrastructure Actually Means

Cold email infrastructure refers to the complete technical foundation that supports your outbound email campaigns. It is not one thing. It is five interconnected components working together:

1. Domains -- Separate domains purchased specifically for outbound email, kept distinct from your primary business domain.
2. Inboxes -- Individual email accounts created on those domains, each acting as a separate sender.
3. Providers -- The email service providers hosting those inboxes, primarily Google Workspace and Microsoft 365.
4. Authentication -- DNS records (SPF, DKIM, DMARC) that verify your emails are legitimate and authorised.
5. Warmup -- The process of gradually building sender reputation before sending any outreach.

Think of it this way: if your cold email campaign is a building, infrastructure is the foundation, plumbing, and electrical wiring. You can have brilliant architecture on top, but if the foundation is cracked, the building will not stand.

Most businesses focus entirely on what prospects see -- the subject line, the copy, the offer. Those matter, absolutely. But they only matter if the email arrives. Infrastructure determines arrival. Everything else determines response.

Here is the question that separates serious outbound operations from amateur ones: are you willing to invest in the part of the process your prospects never see?

Domains -- How Many and Why

Your primary domain -- the one your website sits on, the one your clients email you through -- should never be used for cold email. Full stop.

Why? Because if that domain gets flagged for spam (and with enough volume, some flagging is inevitable), your entire business communication is compromised. Client emails going to spam. Internal communication disrupted. Your brand reputation damaged.

Instead, you purchase secondary domains. These are variations of your brand name that are close enough to be recognisable but separate enough to protect your core domain. For a company called Sparklead, these might look like:

• sparkleadhq.com
• trysparklead.com
• gosparklead.com
• sparkleadmail.com
• hellosparklead.com

How many do you need?

For a serious campaign, you need a minimum of 30 to 50 domains. That number surprises most people, but the logic is straightforward.

Each domain should only host 2 to 3 inboxes. Each inbox should only send 7 to 10 emails per day. If you want to reach enough prospects to generate meaningful pipeline, you need volume. And volume requires distribution across many domains.

This distribution serves two purposes:

1. It protects individual domains. If one domain gets flagged, only 2 to 3 inboxes are affected, not your entire campaign.
2. It keeps per-domain volume low. Email providers are suspicious of new domains sending high volumes. By spreading your sending across dozens of domains, each one looks like a normal business account sending a handful of emails per day.

What do domains cost?

Roughly EUR 10 to 15 per domain per year. For 40 domains, that is EUR 400 to 600 per year -- a relatively minor cost in the context of a lead generation campaign. The mistake is not the cost. The mistake is thinking you can run a serious campaign on 3 to 5 domains.

Inboxes -- Premium vs Cheap

Not all inboxes are created equal. The provider hosting your inbox has a direct impact on your deliverability.

Premium providers: Google Workspace and Microsoft 365

Google Workspace and Microsoft 365 inboxes carry inherently higher sender reputation. These are the same platforms used by legitimate businesses worldwide. When a receiving server sees an email coming from a Google Workspace account, it treats it differently than an email from an unknown or budget provider.

This is not speculation. It is observable in deliverability data. Emails sent from Google Workspace and Microsoft 365 consistently land in primary inboxes at higher rates than emails sent from cheaper alternatives.

Budget providers: shared hosting and discount services

Cheaper inbox providers often use shared IP addresses. That means your emails are being sent from the same IP as potentially hundreds of other senders -- some of whom may be sending genuine spam. Their bad behaviour affects your deliverability. You are guilty by association.

The recommended approach

We use a 50/50 split between Google Workspace and Microsoft 365. This provides two benefits:

1. Diversification. If Google tightens its sending policies (which happens periodically), your Microsoft inboxes continue operating normally, and vice versa.
2. Maximum reputation. Both providers carry strong sender reputation, giving every inbox in your infrastructure the best possible starting position.

Volume per inbox

Each inbox should send between 7 and 10 emails per day. Not 20. Not 50. Seven to ten.

Why so few? Because email providers monitor sending patterns. A brand-new inbox that suddenly starts sending 50 emails per day looks like a spam operation. An inbox sending 8 emails per day looks like a normal professional going about their work.

Total inboxes per client

At Sparklead, we set up 120 to 125 inboxes per client. We ramp each inbox gradually from 5 to 7 emails per day up to a maximum of 15. That gives us the volume to reach thousands of prospects per month while keeping each individual inbox well within safe sending limits.

The question to ask any agency or consultant handling your cold email: how many inboxes are they setting up for you, and what is the per-inbox daily sending volume? If the answer is fewer than 50 inboxes or more than 15 emails per inbox per day, your deliverability is being compromised.

Warmup -- The Non-Negotiable Step

You have your domains. You have your inboxes. You are ready to start sending, right?

No. Not for at least 14 to 21 days.

What warmup is

Warmup is the process of simulating normal email activity on a new inbox before using it for outreach. Warmup tools send and receive emails between your inbox and a network of other inboxes, mimicking the patterns of genuine business communication.

During warmup, your inbox:

• Sends emails that get opened and replied to
• Receives emails that it opens and replies to
• Gradually increases its daily sending volume
• Builds a history of positive engagement signals

Email providers use these signals to establish sender reputation. An inbox with 21 days of positive engagement history is treated very differently from an inbox with zero history.

What happens if you skip warmup

If you skip warmup and start sending cold emails from a fresh inbox, one of two things will happen:

1. Your emails go directly to spam. The inbox has no reputation, so email providers default to treating it as suspicious.
2. Your domain gets burned. Sending volume from an unwarmed inbox triggers spam filters, and the domain itself gets flagged. Once a domain is burned, it is extremely difficult to recover. In most cases, you simply have to abandon it and start again.

Either outcome means wasted money on domains and inboxes, wasted time on campaign setup, and zero results.

How long does warmup take?

A minimum of 14 days. We recommend 21 days. Some providers suggest even longer for larger volumes.

During this period, you cannot send any outreach from those inboxes. This means infrastructure setup needs to begin 3 weeks before you plan to launch any campaign. If an agency tells you they can have you sending within a week of signing up, ask them about their warmup process. If they do not have one, walk away.

Warmup tools

The primary tools for warmup are built into cold email platforms like SmartLead and Instantly. These platforms maintain warmup networks specifically designed to build sender reputation for outbound inboxes. The warmup runs automatically in the background -- you configure it once, and the platform handles the rest.

Authentication -- SPF, DKIM, DMARC

Authentication is the technical verification layer that tells receiving email servers your emails are legitimate. There are three records, and all three are non-negotiable.

SPF (Sender Policy Framework)

SPF is a DNS record that tells receiving servers which IP addresses are authorised to send email on behalf of your domain. When a server receives an email from your domain, it checks the SPF record. If the sending IP is listed, the email passes. If it is not, the email is flagged.

Without SPF, any server in the world could send emails pretending to be from your domain. Receiving servers know this, so they treat emails without SPF verification with heavy suspicion.

DKIM (DomainKeys Identified Mail)

DKIM adds a cryptographic signature to every email you send. The receiving server uses a public key published in your DNS records to verify that signature. If the signature matches, it confirms two things:

1. The email genuinely came from your domain.
2. The email content was not altered in transit.

DKIM is particularly important because it provides a level of verification that SPF alone cannot. SPF verifies the sending server. DKIM verifies the message itself.

DMARC (Domain-based Message Authentication, Reporting, and Conformance)

DMARC ties SPF and DKIM together and tells receiving servers what to do when authentication fails. You can set your DMARC policy to:

• None: Monitor failures but take no action (used during initial setup)
• Quarantine: Send failed emails to spam
• Reject: Block failed emails entirely

DMARC also provides reporting, so you can see how many emails are passing or failing authentication across all your domains.

Why all three matter

Each record addresses a different aspect of email verification. SPF verifies the sender. DKIM verifies the message. DMARC provides the policy and reporting layer. Missing any one of them creates a gap that email providers will notice and penalise.

Setting up authentication correctly takes about 15 to 20 minutes per domain. For 40 domains, that is roughly 10 to 13 hours of DNS configuration. It is tedious, detail-oriented work, and errors are common. But it is absolutely essential.

What Good Infrastructure Costs

Here is the honest breakdown of what proper cold email infrastructure costs per client, per month:

That is a significant monthly investment, and it is entirely separate from the cost of list building, copywriting, and campaign management. This is purely the technical foundation.

Why cheaper options fail

When you see agencies offering cold email services for EUR 500 per month all-in, ask yourself: how are they covering infrastructure costs? The answer is usually one or more of these:

• Fewer inboxes. Instead of 120, they use 20 to 30. Volume is lower, or per-inbox sending is dangerously high.
• Cheap providers. Instead of Google Workspace and Microsoft 365, they use budget providers with shared IPs.
• Skipped warmup. They start sending immediately to show fast results, burning domains in the process.
• Shared infrastructure. Multiple clients send from the same domains and inboxes, so one client's poor list quality damages another client's deliverability.

The cost of infrastructure is not where you want to look for savings. It is the foundation everything else is built on. Cut corners here, and every euro you spend on copywriting, list building, and campaign management is wasted.

The Bottom Line

Cold email infrastructure is not glamorous. It is not the part of outbound that gets discussed in LinkedIn posts or YouTube videos. But it is the single biggest determinant of whether your campaigns succeed or fail.

Before you worry about your subject lines, your offer, or your call-to-action, ask these questions:

• How many domains are you sending from?
• How many inboxes, and what providers?
• What is your per-inbox daily sending volume?
• How long did you warm up before sending?
• Are SPF, DKIM, and DMARC properly configured on every domain?

If you cannot answer all five with confidence, your infrastructure needs attention before anything else.