What is a mailbox pool?
Sending mailboxes on secondary domains, rotated so cold outreach at scale never risks the main domain.
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, cannot be sent safely from one mailbox. Every provider has an informal ceiling on daily sends before it starts throttling or flagging an address, and a single mailbox pushed past that ceiling risks its own reputation fast. A mailbox pool spreads that volume across many mailboxes on domains set up specifically for outreach, rotating sends between them so no single address ever approaches its limit.
The domain choice is what separates a pool that protects a business from one that damages it. Secondary domains, registered and warmed specifically for outreach, are built to absorb the reputation risk cold sending carries; if one takes a hit, the fallout stays contained to that domain. The primary company domain, used for real customer email, should never be part of a pool, and a pool should not double as the address customers reply to for anything other than the outreach conversation itself, since mixing outreach and customer traffic on the same mailboxes puts everyday email at risk from cold-sending mistakes.
How a mailbox pool is used
- 1
Secondary domains are registered
Built for outreach, separate from the company's primary domain.
- 2
Mailboxes are created on those domains
Multiple 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 describe the same thing. Mailbox pool and sender pool both refer to a group of mailboxes sharing outbound volume across secondary domains to protect deliverability.
- Why can't a mailbox pool run on the company's main domain?
- Because a reputation hit from aggressive cold sending would follow every address on that domain, including the ones customers and employees rely on for real email.
- Does every mailbox in a pool need to be warmed the same way?
- Yes. Each new mailbox added to a pool starts with no sending history and needs its own warmup period before it can safely carry a full share of the volume.