$ man redundancy-planning

Redundancy: Why 1:1 Isn't Enough

The principle that having exactly one mailbox per domain with zero spares is a fragile setup. If any single mailbox goes down (reputation hit, bounce spike, provider suspension), you lose that sender permanently until you provision a replacement.

In cold email, things break. Domains get flagged. Mailboxes hit bounce thresholds. Inbox providers suspend accounts for "suspicious activity." If you're running 18 senders with 18 domains and 18 mailboxes (1:1:1), every failure is a permanent capacity reduction. You can't just spin up a new mailbox instantly — it needs 2-3 weeks of warmup before it's usable. Redundancy means you always have warmed, ready mailboxes on standby.

I never run a 1:1:1 setup. Minimum is 1:1:2 (1 domain, 1 active sender, 2 total mailboxes). For critical partner campaigns where downtime isn't acceptable, I go 1:1:3. The extra mailboxes cost a few dollars per month each but save weeks of recovery time when something breaks. The redundancy math: if I need 18 active senders and I want 2x redundancy, I provision 18 domains with 36 mailboxes. If I need 36 active senders with 2x redundancy, it's 36 domains with 72 mailboxes.