Deliverability is the infrastructure layer that determines whether your cold email campaigns produce meetings or vanish into spam folders. No amount of clever copy fixes a broken sender reputation.
This guide covers the operational fundamentals: domain setup, warmup strategy, inbox rotation, and the monitoring signals that matter.
Domain Setup
Never send cold email from your primary domain. Set up dedicated sending domains that are visually related but isolated from your main brand:
- Use variations:
getacme.com,tryacme.io,acmehq.com - Register 3-5 domains per campaign vertical
- Set up SPF, DKIM, and DMARC on day one
- Age domains for at least 2 weeks before sending
Warmup Strategy
Inbox warmup is not optional. Every new mailbox needs 2-3 weeks of simulated engagement before it touches a cold prospect:
- Start with 5-10 warmup emails per day
- Ramp to 30-40 over two weeks
- Maintain warmup alongside live campaigns
- Monitor warmup reply rates — drops signal reputation issues
Inbox Rotation
Sending volume per inbox should stay under 30-40 emails per day. Above that, spam risk increases sharply:
- Calculate: monthly sends ÷ 30 ÷ 35 = inboxes needed
- Example: 7,500 sends/month needs ~7 inboxes minimum
- Add 20% buffer for warmup volume
- Rotate sending across inboxes evenly
Monitoring Signals
Track these metrics weekly:
- Inbox placement rate: What percentage of test emails land in the primary inbox vs. spam?
- Blacklist status: Check against major DNSBLs (Spamhaus, Barracuda, SURBL)
- Domain reputation: Google Postmaster Tools for Gmail-specific signals
- Bounce rate: Keep under 3%. Above 5% is a red flag.
- Reply rate: Healthy cold campaigns see 3-8% positive reply rates
The Stack Decision
Your deliverability monitoring tool is the most important purchase in your cold email stack. Buy the sender layer first, then add sequencing and data on top.
Tools like Mailreach specialize in this layer — spam tests, blacklist monitoring, and warmup network management. Pair it with your sending platform (Smartlead, Instantly) for the full picture.