How many sending domains do I need for cold email?

A common founder setup is one primary domain (never used for cold outreach) plus three or four secondary sending domains, each with one or more warmed mailboxes. Keep each inbox to around 30–40 sends per day and rotate.

The right number scales with volume: add warmed domains and inboxes rather than pushing any single inbox harder. The point of multiple domains is isolation — if one gets a reputation hit, you retire it without touching the rest or your primary.