When you start sending from a new domain, the mailbox providers — Gmail, Outlook, Yahoo — have never heard of you. They have no way of knowing whether you are a legitimate sender or a spammer. Without a proper warm-up your campaigns land in spam, your sender reputation sinks, and worse, your transactional mail starts getting blocked too: order confirmations, password resets, reminders.
A warm-up is the process of building that reputation gradually. It is not optional. It is the difference between years of good deliverability and months spent digging yourself out.
Why it matters
- Sender reputation. Mailbox providers keep a score per domain and per IP. It climbs slowly on good behaviour and drops fast on bad. Once you land on a blocklist like Spamhaus, getting off it takes weeks and structural changes, not an apology.
- Engagement is the primary signal. Gmail watches what share of your mail gets opened, replied to or marked as spam in the first few minutes. If your first send of 5,000 emails only gets a 5% open rate, the algorithm reads that as spam.
- It protects your transactional mail. If marketing and transactional share a domain with no separation, a reputation hit on marketing drags transactional down with it. That means unconfirmed orders, password resets that never arrive, and a direct support cost.
The four-week framework
The standard progression for a new domain. Scale the volumes to your list size — what matters is the rate of growth, not the absolute numbers.
- Week 1 — 50 to 200 sends a day, to your most engaged segment only (anyone who interacted in the last 30 days). Targets: open rate above 40%, bounces under 1%, spam complaints under 0.05%.
- Week 2 — 500 to 2,000 sends a day. Add a second engagement tier (active in the last 60 days). Keep content quality high.
- Week 3 — 5,000 to 10,000 sends a day. Start mixing in lukewarm segments. Check Google Postmaster Tools daily.
- Week 4 — Full volume. Watch bounces, spam complaints, open rate and domain reputation. If anything moves, step the volume back down.
Signals to watch
- Bounce rate under 2%
- Spam complaints under 0.1%
- Open rate above 20% throughout the warm-up
- Google Postmaster Tools reputation at Medium or High, never Low
- Inbox placement above 85% (GlockApps or a similar seed test)
Common mistakes
- Starting at full volume. This is mistake number one. It kills a domain in 48 hours.
- Sending to an old list without cleaning it. If you have not sent in six months, verify it first (Kickbox, NeverBounce). Every hard bounce damages reputation.
- SPF, DKIM and DMARC not set up correctly. Without them, mailbox providers treat you as suspicious by default.
- Mixing transactional and marketing on the same subdomain. Split them: mail.yourdomain.com for marketing, notifications.yourdomain.com for transactional.
- Ignoring Postmaster Tools. It is free and it is Google's own data. Skip it and you are flying blind.
A warm-up done right is four weeks of patience. Done wrong it is months of recovery, plus the revenue lost to blocked transactional mail.