Warmup pools

A warmup pool is a group of mailboxes that send warmup mail to one another. Your mailbox sends a short message to another mailbox in the pool. That mailbox opens it, sometimes replies, and the pattern reverses with mail from another pool member.

What a warmup pool is

From a mailbox provider's point of view, the pattern looks like normal individual mailbox activity: low volume, conversational, mixed senders.

Why pool quality matters more than size

Some tools advertise "5,000+ inboxes in the pool". Sounds impressive. It almost always isn't.

A pool of 5,000 mailboxes mostly assembled from free-tier signups and throwaway providers gives you essentially zero positive signal. Modern mailbox providers either ignore signals from low-trust addresses entirely, or treat them as negative ("why is this mailbox corresponding with so many disposable inboxes?").

A pool of 500 vetted, warmed, monitored mailboxes across the major receivers is dramatically more useful.

What a healthy pool looks like

• Diverse: spans Gmail, Outlook, Yahoo, iCloud, and a representative share of self-hosted IMAP servers.
• Monitored: every member has spam-score tracking; misbehaviour removes the mailbox from the pool.
• Conversational: reply rate inside the pool ~30%; the pool exchanges threads, not one-off blasts.
• Avoidance-aware: the same two mailboxes don't pair up over and over (recent-partner gap of 7 days or more).
• Token-verified: every message carries a single-use token bound to its recipient, so warmup mail cannot be replayed or redirected, and mail arriving in a mailbox is never held against it.

Free vs premium pools

Free pools are useful for testing the mechanics but include higher-risk members. Premium pools should:

• Only include warmed mailboxes meeting reputation criteria.
• Apply stricter quarantine thresholds than free pools.
• Keep a separate recovery pool for senders coming back from quarantine.
• Never silently mix free-tier mailboxes into the premium recipient set.

In Warmbly, free and premium pools are physically separated. Reserved sending infrastructure does not change that: it is for isolation, not a reputation shortcut.

What reserved sending means

Plans that include it reserve a worker for your organisation, so your mailboxes always authenticate to their providers from an address no other customer uses. It gives you:

• A predictable sign-in address for every one of your mailboxes.
• Isolation from another tenant triggering a per-address authentication throttle.
• The ability to negotiate allowlisting with specific partners.

It does not:

• Change the address your recipients see. That belongs to your mailbox provider, not to Warmbly.
• Bypass per-mailbox health rules.
• Let you ignore complaint or bounce thresholds.

Re-entry after quarantine

A quarantined mailbox shouldn't automatically re-enter the pool just because time elapsed. Re-entry should require:

• Healthy SPF, DKIM, DMARC.
• No recent complaints or hard-bounce spikes.
• No recent tampering with received warmup mail.
• Spam-folder placement under 10% on a probation sample.
• Gradual ramp on re-entry, starting at 5–10/day warmup.

Pool quality is shared infrastructure. A platform that lets one bad mailbox keep participating to avoid an awkward support conversation taxes every other paying customer.