The most common way people burn cold email infrastructure is deciding they need more volume and getting it from the inboxes they already have. It works for about three weeks, then placement collapses and they cannot work out why.
Volume comes from breadth. More capacity, more domains, same conservative limits. Here is how to size it.
The formula
Inboxes = daily sends ÷ safe per-inbox limit
Azure tenants = inboxes ÷ 100 Google domains = inboxes ÷ 3
The safe limits are not a preference, they are set by the provider:
- Microsoft Azure: 2 to 3 emails per inbox per day
- Google Workspace: up to 15 emails per inbox per day, 3 inboxes per domain.
On Azure the tenant is the unit: one .com domain with 25, 50 or 100 inboxes for €30, which is why the domain is bundled into the price. Google runs 3 inboxes per domain, so a Google estate needs far more domains.
Worked examples
| Daily sends | Azure inboxes | Azure tenants (= domains) | Google inboxes | Google domains |
|---|---|---|---|---|
| 500 | 200 | 2 | 35 | ~12 |
| 1,000 | 400 | 4 | 70 | ~24 |
| 2,000 | 800 | 8 | 135 | 45 |
| 6,000 | 2,400 | 24 | 400 | ~134 |
Azure needs more inboxes but far fewer domains, because each tenant runs on one .com. Google needs fewer inboxes but many more domains, each bought separately. At €30 a tenant with the .com included, 4 Azure tenants (€120) cost less than 70 Google inboxes (€210) across 24 domains you have to register yourself.
Warm-up changes the timeline, not the maths
New inboxes do not send at full volume on day one. The ramp differs by provider:
Days 1 to 14: 5 warm-up emails a day, increasing by 1 per day, target reply rate 65 percent. From day 15: 2 to 3 cold emails a day with warm-up maintained at 5, and 60 to 80 minutes between sends.
Sending limit 15 a day, warm-up limit 20 a day increasing by 2, reply rate 35 percent, 20 minutes between sends.
These are not interchangeable. Applying Google's numbers to an Azure tenant is the fastest way to lose it.
Pre-warmed inboxes skip this. They have already had four weeks of warm-up and are delivered and sending within 24 hours, which matters if you are replacing a set mid-campaign.
Sizing for growth, not for today
Order for the volume you want in two months, not the volume you need this week. Warm-up takes two weeks and provisioning takes 24 hours, so infrastructure ordered today is at full capacity in roughly a fortnight.
The other thing to plan for is decay. Sets do not last forever, and reputation drifts down over months. Running a rotation means a replacement set is ordered at week four and takes over at week six, so you always have a warmed set ready rather than scrambling when numbers dip.
Want this done for you?
Our Build and Release service stands up your entire cold email infrastructure: domains, mailboxes, authentication, warm-up, and tracking, all authenticated and ready to send. You skip the weeks of setup and start with campaigns.
Book a call