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 verification token. A message whose token fails verification earns no warmup credit. If it is this mailbox's own warmup mail seen late, it is still filed out of the inbox; anything else is handled as ordinary mail.

Open vs vetted pools

Open pools, the kind anyone can join for free, are useful for testing the mechanics but include higher-risk members. A vetted pool should:

• Only include warmed mailboxes meeting reputation criteria.
• Apply stricter quarantine thresholds than open pools.
• Keep a separate recovery pool for senders coming back from quarantine.
• Never silently mix unvetted mailboxes into its recipient set.

In Muster CRM there is one pool: the mailboxes connected to this deployment, across all of its workspaces. None come from outside it, so its diversity is the mix of providers you connect.

What "dedicated" should mean

Dedicated workers are not something you buy on this deployment; this is what the term should mean when a sending service offers one. A dedicated worker is a worker process and IP allocated to your organisation. It gives you:

• Predictable sending IP.
• Isolation from other tenants' bad days.
• The ability to negotiate IP allowlisting with specific partners.

It does not:

• Bypass per-mailbox health rules.
• Let you ignore complaint or bounce thresholds.
• Magically warm a brand-new IP. Dedicated IPs need their own warmup ramp.

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 invalid warmup-token attempts.
• 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 mailbox in the pool.