$ man per-inbox-limits

Per-Inbox Limits

The maximum number of emails a single mailbox should send per day. Not a platform-imposed limit (though those exist) — a self-imposed limit based on what delivers reliably without triggering spam filters.

Every mailbox has a reputation. Push it too hard and the reputation drops. Once a mailbox is flagged for high volume, every email from it gets scrutinized. Per-inbox limits protect individual mailbox reputation. It's better to spread volume across many mailboxes at low limits than concentrate volume in a few mailboxes at high limits.

I set per-inbox limits in Instantly's campaign settings. Google mailboxes: 25-30/day max. Microsoft mailboxes: 10-20/day max (depending on warmup stage). These limits are non-negotiable. If a partner needs higher total volume, I add more mailboxes instead of raising per-inbox limits. The math is simple: 36 mailboxes × 25/day = 900 emails/day. That's enough for almost any campaign. More mailboxes, not more emails per mailbox.