The “3 Inboxes per Domain” Rule: How to Scale Without Burning ...

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Every place a page here uses this source, in the order the words come in it.

1. The rule is straightforward: “For each sending domain (e.g., getacme.com), create no more than 3 active sending mailboxes (e.g., alex@, jordan@, sam@).” Use those mailboxes for outbound sending only.

   In Domain and mailbox architecture

2. For each sending domain (e.g., getacme.com), create no more than 3 active sending mailboxes (e.g., alex@, jordan@, sam@). “Use those mailboxes for outbound sending only.” When you need more volume, add another domain (e.g., tryacme.com) and repeat.

   In Domain and mailbox architecture

3. Use those mailboxes for outbound sending only. “When you need more volume, add another domain (e.g., tryacme.com) and repeat.” This is not a law of physics; some teams run 5 inboxes per domain and survive. But if your priority is stable deliverability, predictable inbox placement, and low domain churn, 3 is a safer ceiling. Why? Because deliverability isn’t just about what one mailbox does. It’s about what the domain and infrastructure signal to mailbox providers.

   In Domain and mailbox architecture

4. “That’s where the “3 inboxes per domain” rule comes in.” Cap at ~3 sending inboxes per domain (especially early on)

   In Mailbox provisioning

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