Why fresh inboxes land in spam (and the 21-day fix)

A brand-new mailbox has no sender history. To Gmail or Microsoft, no history means no trust — so your first sends default to the spam folder, no matter how good the copy is.

What ESPs actually measure

Filters grade behaviour, not promises: are your messages opened, replied to, marked important, moved out of spam? One-way blasts to a seed list are easy to discount. Real conversations are not.

The 21-day curve

Reputation starts building within ~24 hours, but most accounts stabilise around day 14–21. A ramp that starts slow and grows daily keeps you under each provider’s cap while the engagement signal accumulates.

Before you warm up

Make sure SPF, DKIM and DMARC are in place — our free tools generate the records. Broken DNS caps your score before the first send.

Connect a mailbox and WarmupSleuth runs this loop for you — free during the open Beta.