$ man domain-to-sender-ratio

Domain-to-Sender Ratio

The relationship between the number of domains and the number of sending mailboxes. At minimum, 1 domain per sender (18 senders = 18 domains). For redundancy, 1 domain with 2 mailboxes (18 senders = 18 domains, 36 mailboxes).

If you put 3 senders on 1 domain and that domain gets flagged, you lose 3 senders at once. A 1:1 domain-to-sender ratio limits blast radius — one domain flagged means one sender offline. Adding a second mailbox per domain (1:2 ratio) gives you a backup. If one mailbox's reputation dips, you rotate to the second while the first recovers. Redundancy isn't waste — it's insurance.

My standard setup is 1 domain with 2 mailboxes. So if I need 18 active senders, I provision 18 domains with 36 mailboxes — 18 primary, 18 backup. The primary mailboxes send in the current campaign. The backups warm passively. If a primary mailbox's deliverability drops, I swap in the backup and let the primary rest. This rotation happens without pausing the campaign. If I'm running 36 active senders, I provision 36 domains with 72 mailboxes.