Most outbound teams I audit have the same infrastructure mistake: they set up too many inboxes on too few domains, skip the warmup, and start sending at volume from day one. Then they wonder why their open rates are 8% and their deliverability is wrecked before they've sent a single campaign worth sending.
Cold email infrastructure is not exciting. It's also the thing that determines whether your carefully written sequence actually lands in the inbox or in spam. Get this right before you write a single email.
The 3-Inbox-Per-Domain Rule
The safe default for cold email infrastructure is 3 inboxes per sending domain. Not 5, not 10. Three.
Here's why: Google and Microsoft evaluate sender reputation at the domain level, not the individual inbox level. If one inbox on your domain gets flagged for spam — because a prospect marked it, or you hit a spam trap — that reputation signal affects every other inbox on the same domain. Spreading three inboxes across a domain limits your blast radius when one inbox has a bad day.
When you put 5 or more inboxes on a single domain, you're concentrating risk. A spam spike from any one of those inboxes damages the domain reputation that all of them share. I've seen teams burn through a 6-month-old domain in two weeks because they overloaded it with inboxes and sent too fast.
How Many Emails Per Inbox Per Day?
After completing warmup, a single inbox in steady state should send 30 cold emails per day. Some experienced operators push to 50/day on inboxes with strong reputation histories, but 30 is the conservative safe limit that keeps you well below spam rate thresholds for both Google and Microsoft.
The ramp during warmup:
- Week 1: Start at 10 emails per inbox per day, alongside warmup tool sends
- Week 2: Increase to 15–20 per inbox per day
- Week 3: Increase to 25 per inbox per day
- Week 4+: Full send at 30/day steady state
Do not send cold emails from a brand-new inbox before completing at least 2–3 weeks of warmup. A new inbox with no sending history that suddenly sends 30 cold emails on day 1 is a pattern email providers are trained to flag as spam.
Warmup tools that work well in 2026: Instantly, Smartlead, and Mailreef. All three maintain networks of real inboxes that exchange emails with your new inbox, building sending reputation before you send a single cold email. Most are included in the subscription cost of major cold email platforms.
The Math to Calculate Your Infrastructure Need
The formula is simple once you know your daily send target:
Worked example: You want to send 300 cold emails per day.
- 300 ÷ 30 = 10 inboxes needed
- 10 ÷ 3 = 4 domains needed (round up — don't put more than 3 inboxes on any domain)
For a team of 3 SDRs each sending 300 emails/day (900 total):
- 900 ÷ 30 = 30 inboxes
- 30 ÷ 3 = 10 domains
This is more domains than most teams expect. The cost of domains is low ($10–15 each per year). The cost of burning a domain through over-sending is the loss of months of warmup work.
Buying and Setting Up Domains
A few hard rules on domain purchases:
- Buy from Namecheap or Cloudflare, not GoDaddy. GoDaddy's shared nameserver infrastructure has historically had deliverability issues and their domain management UI is genuinely painful.
- Use .com or your country TLD. Exotic TLDs (.xyz, .io used carelessly, .co used without track record) have higher spam association in email provider filters. .com is still the cleanest.
- Choose domain names adjacent to your brand. If your company is Acme Corp (acmecorp.com), your sending domains might be: acmehq.com, useacme.com, acmegrowth.com. Don't use random unrelated names.
The DNS Setup Checklist
- SPF record configured (authorizes your sending IP)
- DKIM record configured (cryptographic email signing)
- DMARC record configured (policy: at minimum p=none to start, p=quarantine after 30 days)
- Custom tracking domain set up (never use the sequencing tool's default tracking domain)
- MX records pointing to Google Workspace or Microsoft 365
Configure DNS records before you connect the inbox to your sequencing tool, and before starting warmup. Many teams start warmup before DMARC is in place — this limits the reputation signals your domain builds during warmup.
Warmup — The Step Most People Skip
Warmup is the process of gradually building a sending reputation for a new inbox by exchanging emails with other real inboxes. It mimics legitimate email behavior — which teaches Google and Microsoft that this inbox is used by a real human who sends and receives normal correspondence.
What warmup actually does: the warmup tool connects your inbox to a network of other real inboxes. Those inboxes send your inbox emails, and your inbox sends them emails back. Those emails are automatically opened, replied to, and occasionally moved out of spam — all behaviors that build positive reputation signals for your domain and inbox.
Minimum warmup period: 2 weeks. Recommended: 3 weeks. For high-volume campaigns (>100 sends/inbox/day), consider 4 weeks of warmup before touching cold sends.
Run warmup concurrently with cold sends — don't stop warmup once you start sending. Warmup signals continue to protect your domain reputation even while the inbox is active. Most tools handle this automatically.
Monitoring and Maintenance
Setting up infrastructure is a one-time cost. Maintaining it is ongoing. What to monitor monthly:
- Google Postmaster Tools. Free tool that shows your domain reputation, spam rate, and delivery errors for sends to Gmail addresses. Spam rate above 0.3% is a warning. Above 0.5% is a serious problem that requires immediate volume reduction.
- Microsoft SNDS (Smart Network Data Services). Equivalent monitoring for sends to Outlook/Hotmail addresses. Check monthly — problems here usually surface before they show up in reply rate data.
- Domain rotation. Any domain that sustains a spam rate above 0.3% for two consecutive weeks should be paused from cold sends and moved into warmup-only mode for 30 days. Don't try to muscle through with a damaged domain — rotate it out and bring in a fresh one.
Never use your primary company domain (the one your company email runs on) for cold email sends. If a cold email campaign damages that domain's reputation, it affects your transactional emails, customer communications, product notifications, and everything else that depends on email deliverability.
Calculate Your Infrastructure Requirements
Use the Cold Email Calculator to model how many domains and inboxes you need based on your target daily send volume, sequence length, and campaign timeline.