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 inboxes, more domains, same conservative per-inbox limits. Here is how to size it.
The formula
Inboxes = daily sends ÷ safe per-inbox limit
Azure domains = inboxes ÷ 100 Google domains = inboxes ÷ 3
The safe per-inbox limit is not a preference, it is set by the provider:
- Microsoft Azure: 2 to 3 emails per inbox per day
- Google Workspace: up to 15 emails per inbox per day
Inboxes per domain is not one number, it is provider specific. An Azure tenant of 100 accounts runs on a single domain, which is why the .com is bundled into the per-inbox price. Google runs 3 inboxes per domain, so a Google estate needs far more domains than an Azure one of the same inbox count.
Worked examples
| Daily sends | Azure inboxes | Azure 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 roughly five times the inboxes but far fewer domains, because 100 accounts sit on one domain. Google needs fewer inboxes but many more domains, each bought separately. At €0.30 per inbox with the .com included, 400 Azure inboxes across 4 domains cost less than 70 Google inboxes 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 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 sending within about six hours of arriving, 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.
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