Cold Email
Infrastructure
From small campaigns to enterprise scale — everything you need to set up, run, and protect your sending infrastructure.
The Fundamentals
Infrastructure is not the platform you use to send emails. It is the foundation that determines whether those emails actually reach inboxes. Get this wrong and nothing else matters.
Your main domain underpins every email your team sends — internal, client-facing, transactional. If it gets flagged due to cold outreach, you damage everything.
Rule: Never send cold outreach from your primary domain. Always use dedicated secondary domains.
| Record | What it does | Why it matters |
|---|---|---|
| SPF | Authorizes which servers can send on your behalf | Without it, emails are flagged as unauthorized |
| DKIM | Cryptographic signature proving email wasn't altered | Required by Gmail, Yahoo, and Outlook for bulk senders |
| DMARC | Policy for what happens when SPF or DKIM fail | Protects domain identity and improves trust score |
Configure SPF and DKIM first, wait 48 hours, then set up DMARC. Target a mail-tester.com score of 9+ before sending anything.
Domain & Inbox Setup
.com only — avoid .co, .tv, .tech, .agency or any non-standard TLD. No hyphens, underscores, or numbers.
- Primary domain (never used for outreach): energyleads.co
- Sending examples: energysalesleads.com · leadsenergyagency.com · energyleadsales.com
- ~$20/domain + $5–10/month per inbox. GoDaddy or Porkbun.
| Option | What it is | Best for | Tools |
|---|---|---|---|
| Private | Dedicated IP and sending environment | High volume, agencies, max control | Mailforge, Infraforge |
| Hosted | Google Workspace or M365 | Easier setup, SMB / startup targeting | Maildoso, Google Workspace |
| Hybrid | Mix of both for diversified risk | Scaling teams | Mailforge + Maildoso |
Use both. Private outperforms for large corporates; hosted performs better for SMBs. Diversification also protects against rule changes by any single provider.
| Domains | Inboxes (3 per domain) | Max emails/day |
|---|---|---|
| 1 | 3 | 90 – 150 |
| 5 | 15 | 450 – 750 |
| 10 | 30 | 900 – 1,500 |
| 20 | 60 | 1,800 – 3,000 |
| 50+ | 150+ | 4,500 – 7,500+ |
Always have additional domains warming in the background. When a domain gets flagged, you need a healthy replacement ready immediately.
Warmup
A fresh domain that suddenly sends dozens of cold emails will be flagged immediately. Warmup gradually builds sender reputation so inbox providers treat the domain as trusted.
| Period | Daily volume per inbox | Focus |
|---|---|---|
| Days 1–7 | 5 – 10 emails | Building baseline reputation |
| Week 2 | 10 – 20 emails | Establishing consistent sending pattern |
| Weeks 3–4 | 20 – 35 emails | Steadily increasing, monitoring placement |
| Weeks 5–6 | 35 – 50 emails | Approaching full operational capacity |
| Week 7+ | Up to 50 max | Full outreach — keep warmup running always |
Rule: Never exceed 50 cold emails/day from a single inbox. Most high-performing teams target 20–30/day. Warmup should represent at least 15% of total sending volume at all times.
- Use a dedicated warmup tool — do not rely on manual warmup
- Avoid automated engagement pods — they're detectable by inbox providers
- Maintain consistent daily volumes — erratic patterns damage reputation
- Minimum 3 weeks before any cold campaign. 4–6 weeks is ideal
- Keep warmup running continuously even after full ramp-up
Sending Limits & Thresholds
| Metric | Safe threshold | Action if exceeded |
|---|---|---|
| Cold emails per inbox/day | Max 50 (target 20–30) | Immediately reduce volume |
| Hard bounce rate | Under 2% | Pause, re-verify the list |
| Spam complaint rate | Under 0.1% (0.3% enforced max) | Stop campaign, audit copy + targeting |
| Reply rate benchmark | 3% to 8% | Below 1% = targeting or copy issue |
- Microsoft 365 / Outlook: better when targeting large corporates and enterprises
- Google Workspace / Gmail: better when targeting SMBs, startups, and founders
Disable open tracking on all cold campaigns. Tracking pixels are increasingly flagged by spam filters. Reply rates are the only metric that matters.
Scaling the Infrastructure
- 2–5 secondary domains
- 2–3 inboxes per domain
- Full warmup before sending
- Instantly or Smartlead
- Maildoso or Mailforge
- 8–20 domains, 3 inboxes each
- Mix hosted + private infra
- 3–5 fresh domains warming always
- Verify lists before import
- Postmaster Tools weekly
- 20–50+ domains, dedicated IPs
- Automated domain rotation
- Custom CNAME per domain
- GlockApps placement testing
- 10+ domains warming at all times
Scale math: 1,000 emails/day = 12 domains × 3 inboxes = 36 inboxes × 30 emails. 3,000/day = 33 domains × 99 inboxes. Plan 4–6 weeks ahead for warmup.
Monitoring & Maintenance
| Tool | What to monitor | Frequency |
|---|---|---|
| Google Postmaster Tools | Domain reputation, spam rate, delivery errors | Weekly |
| MXToolbox | SPF/DKIM/DMARC validity, blacklist status | Weekly |
| Mail-tester.com | Overall health score (target 9+) | Before each campaign |
| GlockApps | Inbox placement across providers | Monthly / when issues arise |
| Sending platform dashboard | Bounce rate, reply rate, complaint rate | Daily while live |
- Bounce rate above 2% — stop and re-verify immediately
- Sudden reply rate drop with no copy changes — likely a deliverability issue
- Spam complaints spiking — audit targeting and opt-out handling
- Domain on a blacklist — retire it, switch to a pre-warmed replacement
- Inbox placement below 80% — re-warm and reduce volume
Recommended Toolstack
| Category | Tool | Notes |
|---|---|---|
| Domain registration | GoDaddy / Porkbun | .com only |
| Private infra | Mailforge / Infraforge | Scale + dedicated IPs |
| Hosted infra | Maildoso | Google + Outlook, faster setup |
| Email sending | Instantly.ai | Best warmup + campaign combo |
| Email sending (alt) | Smartlead | Strong multi-inbox rotation |
| Email sending (alt) | Lemlist | Good for multichannel sequences |
| Warmup | Instantly / Lemwarm / Mailreach | Run continuously |
| Email verification | Neverbounce | Verify all lists before import |
| Catch-all verification | Scrubby | Layer on top of Neverbounce |
| Enrichment | Bettercontact | Waterfall email + phone enrichment |
| DNS checking | MXToolbox | SPF, DKIM, DMARC, blacklists |
| Deliverability testing | Mail-tester.com | Score 9+ before sending |
| Placement testing | GlockApps | Inbox vs spam across providers |
Pre-Launch Checklist
Tick every box before sending anything. No exceptions.
Deliverability is not a one-time setup. Check metrics weekly, keep fresh domains warming at all times, and treat your domain reputation as a core business asset.