A brand-new sending domain has no reputation with mailbox providers. Send too much, too fast, to a cold list and you'll land in spam — or trip the limits that pause sending entirely. "Warming up" means ramping your volume gradually so providers learn to trust you.

The two limits that decide everything

Posthawk's sending infrastructure enforces account-level reputation limits, and mailbox providers apply similar logic. Stay under these:

• Bounce rate — keep under 5%. Only hard bounces (nonexistent addresses) count. At ≥5% your account is placed under review; at ≥10% sending can be paused. Posthawk auto-suppresses hard bounces so they can't recur.
• Complaint rate — keep under 0.1%. This is recipients marking your mail as spam. The bar is tight — one in a thousand. Complaints are auto-suppressed too.

Watch both live in the dashboard (Metrics) or via the Reputation API (GET /reputation/metrics). You'll get a heads-up in the dashboard before you approach either limit.

A warm-up ramp you can follow

Start small, send to your most engaged contacts first, and roughly double daily as long as bounces stay under 5% and complaints under 0.1%. Hold or pull back a step if rates climb.

Day 1      ~500      most engaged recipients only
Day 2      ~1,000    recent openers / active users
Day 3      ~5,000    watch bounce + complaint rates
Day 4      ~10,000   hold if rates climb
Day 5–7    25k → 50k double daily if clean
Week 2+    full volume — ramp steadily to target

These are guidelines, not hard rules — the right pace depends on list quality. The principle is constant: gradual, engaged-first, and watch your metrics.

Do

• Only send to people who explicitly opted in
• Send your most engaged contacts first — opens build reputation
• Authenticate with SPF, DKIM, and DMARC (Posthawk sets DKIM + SPF for you on domain verification; add a DMARC record too)
• Keep a visible, working unsubscribe — Posthawk adds a one-click List-Unsubscribe header automatically on broadcasts
• Clean your list: remove role addresses, typos, and long-inactive contacts
• Increase volume gradually and watch your bounce + complaint rates

Don't

• Buy or scrape email lists — the fastest way to get blocked
• Blast a large cold list on day one from a brand-new domain
• Ignore bounces — repeated hard bounces wreck your reputation
• Re-send to addresses that have hard-bounced or complained
• Use misleading subject lines or hide the unsubscribe link
• Suddenly 10× your volume — providers throttle sudden spikes

Do I need to warm up IPs?

No. On the cloud edition you send from professionally-managed, pre-warmed shared IP addresses, so there are no IPs for you to warm up — you only ramp your *volume* and list as above. High-volume senders can add managed dedicated IPs (an add-on): they're allocated and automatically warmed up per mailbox provider, spilling overflow to the shared pool during warm-up to protect your new IPs — so even dedicated IPs need no manual ramp. Dedicated IPs only make sense once you're sending consistently high volume (hundreds+ per day); below that, shared IPs deliver better.

A full visual guide lives at https://posthawk.dev/email-warmup