What is a mailbox pool?
Sending mailboxes on secondary domains, rotated so cold outreach at scale never risks the main domain.
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Mailbox pool — A mailbox pool is a set of sending mailboxes across secondary domains that a system rotates through to keep each mailbox under provider volume limits. It is the standard way to send cold outreach at scale without burning a primary domain.
Also known as: sender pool
Why it matters
Cold outreach at real volume, hundreds or thousands of contacts a week, can't go out safely from one mailbox. Every provider has an unwritten cap on daily sends. Past it, the provider starts slowing or flagging the address, and one mailbox pushed over that cap burns its own reputation fast. A mailbox pool spreads the volume across many mailboxes on domains set up just for outreach. Sends rotate between them, so no single address gets near its limit.
Your choice of domain decides whether a pool protects the business or hurts it. Secondary domains, bought and warmed just for outreach, are there to soak up the risk cold sending carries. If one takes a hit, the damage stays on that domain. Your main company domain, the one real customers write to, should never be part of a pool. Nor should a pool double as the address customers reply to for anything beyond the outreach thread itself. Mix the two kinds of mail on the same mailboxes and a cold-sending mistake can hit your everyday email.
How a mailbox pool is used
- 1
Secondary domains are registered
Built for outreach, kept apart from the company's main domain.
- 2
Mailboxes are created on those domains
Many addresses, spread across one or more secondary domains.
- 3
Each mailbox is warmed
Building sending history before it carries real outreach volume.
- 4
Sends rotate across the pool
Each message goes out from whichever mailbox is furthest under its daily cap.
- 5
Struggling mailboxes are pulled from rotation
So one address's problem does not spread to the rest of the pool.
The mistake to watch for
Questions
- How is a mailbox pool different from a sender pool?
- They're the same thing. Both names mean a group of mailboxes on secondary domains that share outbound volume, so each one keeps a good sending record.
- Why can't a mailbox pool run on the company's main domain?
- A reputation hit from heavy cold sending would follow every address on that domain. That includes the ones your customers and staff rely on for real email.
- Does every mailbox in a pool need to be warmed the same way?
- Yes. Each new mailbox in a pool starts with no sending history. It needs its own warmup before it can safely carry a full share of the volume.