Cold email infrastructure fails in predictable ways: too few inboxes for the target volume, no ramp, or a brand-new domain asked to send like a five-year-old corporate mail system.
Warmup is how you avoid that.
What warmup actually is
Warmup is the gradual increase of sending volume from an inbox (and domain) so providers learn that the mailbox is used by a real sender, not a blast machine turned on overnight.
Typical pattern:
- Start low (e.g. 5–15 emails/day)
- Increase every few days
- Cap at a sustainable per-inbox limit (often ~30–50/day for cold, depending on provider and reputation)
Exact numbers vary. The principle does not: sudden volume is a spam signal.
Sizing the fleet
You need enough inboxes so that target daily volume ÷ safe per-inbox cap is covered — then add headroom.
Example:
- Target: 3,000 cold emails/day
- Cap: 40/inbox/day
- Inboxes needed: 75
- If you put 3 inboxes per domain → ~25 domains
The email warmup calculator does this math for you: inboxes, domains, ramp weeks, and rough cost.
Domains and secondary domains
Do not put all outbound on your primary corporate domain if you can avoid it. Secondary domains absorb risk.
When choosing secondaries:
- Prefer names close to the brand so the from-address still feels coherent
- Check availability before you buy
The secondary domain finder generates lookalikes and checks live DNS availability.
Authentication before volume
Warmup will not save a domain with broken SPF or no DMARC. Set authentication first:
Content while warming
Even during warmup, avoid spam-heavy copy. Score subjects and bodies with the subject line checker and spam word checker. Warmup builds trust; bad content spends it.
A sane sequence
- Buy/configure domains and inboxes
- Authenticate (SPF, DKIM, DMARC)
- Warm on a schedule — not a spike
- Verify lists before scaling
- Increase volume only while bounce and complaint rates stay healthy
If you want the fleet designed and provisioned instead of assembled ad hoc, start a build.