What's in this guide
Infrastructure by the numbers
1
Buy sending domains — never use your main domain
This is the most important rule in cold email infrastructure: never send cold outreach from your primary domain. When a domain gets flagged as spam — and eventually it will — every email from that domain is affected. That includes replies from actual customers.
Buy secondary sending domains on Namecheap. They cost $8–12 per year for .com. Register variations of your brand:
Buy 3–5 sending domains upfront. Running one account per domain, that's 225–375 emails per day at full warmup. Scale by adding more domain + account pairs, not by pushing individual accounts harder.
2
Set up Outlook / Microsoft 365 accounts — not Gmail
Use Microsoft 365 (formerly Office 365) accounts. Not Gmail. Not Zoho. Outlook.
Why Outlook specifically? Outlook-to-Outlook email delivery bypasses many of Google's spam filters. A large percentage of B2B targets use Microsoft email. Outlook also allows higher sending volumes per account without triggering rate limits that Gmail imposes on cold outreach.
- Go to Microsoft 365 admin center
- Purchase Microsoft 365 Business Basic ($6/user/month)
- Add your sending domain (e.g., yourdomain.com)
- Create one mailbox per domain (e.g., you@yourdomain.com)
- Enable SMTP AUTH for the mailbox (required for Instantly to send)
Microsoft disables SMTP AUTH by default on new accounts. In the Microsoft 365 admin center: Users → Active Users → select the mailbox → Mail tab → Manage email apps → enable Authenticated SMTP.
3
Configure SPF, DKIM, DMARC — the exact DNS records
Three DNS records. All three required. Missing any one of them tanks deliverability significantly — many receiving servers reject or spam-folder email that fails these checks.
SPF Record
Tells receiving servers which mail servers are authorized to send from your domain.
DKIM Record
Microsoft generates your DKIM keys. Get them from: Microsoft 365 Admin Center → Settings → Domains → select domain → DNS records. Copy the CNAME records exactly:
DMARC Record
Tells receiving servers what to do when SPF/DKIM fail. Start with p=none to monitor only, then move to p=quarantine after 2 weeks.
Use MXToolbox to verify SPF, DKIM, and DMARC are resolving correctly before moving to warmup. DNS propagation takes 1–48 hours.
4
Connect accounts to Instantly.ai
Instantly.ai Hypergrowth ($97/month) is the campaign engine. It handles warmup, sending, sequences, unified inbox, and basic analytics. Connect your Outlook accounts via SMTP/IMAP.
- In Instantly: go to Email Accounts → Add Account
- Select SMTP/IMAP (not OAuth — use app password)
- SMTP host:
smtp.office365.com, port 587, TLS - IMAP host:
outlook.office365.com, port 993, SSL - Username: your full email (you@yourdomain.com)
- Password: Microsoft 365 account password (or app password if MFA enabled)
Enable warmup immediately after connecting the account — before you set up any campaigns. The warmup starts working from day one and you can't get those days back.
5
Run the warmup protocol — 14–21 days minimum
New email account = zero reputation. ISPs are looking for sudden sending spikes from new addresses. The warmup process builds sending reputation by having your account exchange emails gradually with other accounts in Instantly's warmup network.
| Period | Warmup emails/day | Campaign emails/day | Status |
|---|---|---|---|
| Days 1–7 | 5–10 | 0 | Warmup only |
| Days 8–14 | 15–20 | 0 | Warmup only |
| Days 15–21 | 20–30 | 0–10 (cautiously) | Transition |
| Day 22+ | 30 (ongoing) | Up to 45 | Active |
| Month 2+ | 30 (ongoing) | Up to 75 | Full scale |
Keep warmup running even after you start campaigns. It's not a one-time thing — ongoing warmup emails maintain your reputation score. Turn it off and reputation decays over weeks.
6
Write sequences that convert
Infrastructure gets your email delivered. Copy gets it replied to. A basic 3-step sequence:
- Email 1 (Day 1): AI-personalized first line + 2-sentence pitch + soft CTA. Under 100 words total.
- Email 2 (Day 3): Follow-up bump. One line: "Wanted to make sure this didn't get buried." No re-pitch.
- Email 3 (Day 7): Final "breakup" email. "I'll assume now's not the right time — feel free to reach out when it is." Often gets the most replies.
For AI personalization of first lines, see the full guide on AI cold email systems — specifically how Claude AI generates unique openers per prospect.
7
Monitor deliverability — what to watch
Set up mail-tester.com — send a test email there before launching any campaign. Score should be 9/10 or higher.
Key metrics to monitor weekly in Instantly:
- Open rate: Should be 40%+ with Outlook accounts. Below 30% = deliverability problem.
- Bounce rate: Keep below 3%. Above that, stop and clean your list.
- Spam complaints: Zero tolerance. One or two complaints per 1,000 emails = pause campaign.
- Warmup health score: Should stay above 85 in Instantly dashboard.
If open rate drops below 25% suddenly: pause the account, let warmup run for a week, test again. Don't push through bad deliverability — it compounds.
For a full review of Instantly.ai and how it handles these metrics, read the Instantly.ai review 2026. To see how this infrastructure fits into a complete AI outbound system, see AI sales automation.
When this setup doesn't apply
- Transactional email. This infrastructure is for outbound cold email only. For order confirmations, password resets — use a dedicated transactional provider like Postmark or SendGrid.
- Newsletter/marketing to opted-in lists. Use MailerLite, Klaviyo, or similar. Cold email infrastructure is built for outbound; it lacks the unsubscribe management and compliance features newsletters need.
- Very high volumes (10,000+ per day). At that scale, you need dozens of accounts and domains, plus dedicated IP sending infrastructure. Instantly.ai Hypergrowth has limits.
- B2C outreach (personal inboxes). GDPR and CAN-SPAM create significant legal risk for emailing personal (non-business) contacts without consent. This guide is for B2B only.
Frequently Asked Questions
Why should I use a separate domain for cold email?
If your sending domain gets flagged as spam — which happens — it affects all email from that domain, including replies from real customers and internal communication. Always use secondary sending domains so your main domain reputation stays clean.
Why Outlook over Gmail for cold email?
Outlook-to-Outlook email delivery avoids many of Gmail's spam filters. Google is aggressive about filtering cold outreach, especially from non-Gmail senders. Microsoft 365 accounts also allow higher sending volumes per account and integrate cleanly with Instantly.ai.
What DNS records do I need for cold email deliverability?
You need three records: SPF (TXT record telling receiving servers which IPs can send from your domain), DKIM (cryptographic signature proving email authenticity), and DMARC (policy telling receiving servers what to do with emails that fail SPF/DKIM checks). All three are required — missing any one tanks deliverability.
How long does cold email warmup take?
14–21 days minimum. Start at 5 emails/day and scale gradually: 5 → 10 → 20 → 40 → 75 max per account per day. Never skip warmup on a new account. Impatience here burns the domain and you start over.
How many cold emails can I send per day per account?
Maximum 75 emails per day per account after full warmup. To scale volume, add more accounts and domains rather than pushing any single account past this limit.
What is the cheapest way to set up cold email infrastructure?
Minimum viable setup: 1 Namecheap domain ($10/year), 1 Microsoft 365 account ($6/month), Instantly.ai Hypergrowth ($97/month). Total first month: ~$113. This gets you one warmed account capable of sending 75 emails/day — about 1,500 contacts per month.
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