What domain warmup is, and why it exists
Reputation in email is not global — mailbox providers track it per sending domain and per sending IP. A brand-new domain has no track record, so providers treat it with suspicion: send a few hundred messages one day and a hundred thousand the next, and the spike reads like a compromised account or a spammer. Warmup is the fix. You start at low volume and raise it in steady steps, giving Gmail, Yahoo, and Microsoft time to see that real people open, read, and reply. The track record you build is the reputation every later campaign rides on.
How long warmup takes
Plan for roughly two to six weeks, not a weekend. Amazon SES says a new IP can earn a positive reputation in about two weeks with some providers and up to six with others, and its automatic warmup runs on a fixed 45-day schedule. Postmark's guidance runs along the same lines — start very low and grow incrementally, with most domains reaching dependable full-volume deliverability in a few weeks. The larger your target volume, the longer it takes, because each provider raises the ceiling on how much unfamiliar mail it accepts only as your numbers hold up.
Send your most engaged contacts first
Order your sends by recency of engagement. Your first warmup mail should go to the people most likely to open, click, and reply — recent signups, active users, anyone who touched your product this week. Postmark and Amazon SES make the same point: early positive signals from your most active users build reputation faster and keep complaints low, which is what convinces a provider to raise your limit. Work down from there — recently engaged, then older-but-active — and leave dormant or never-opened contacts until the domain is established, if you mail them at all. A single wave to a stale list early on can undo a week of careful ramping.
Warm the domain — and the IP if it's dedicated
Reputation rides on two things at once: the sending domain and the IP address it goes out on. On a shared IP — the default at most providers — you sit in a pool with other senders, and the pool's established reputation cushions a new domain, so you mainly warm the domain. A dedicated IP is yours alone, which gives you full control but also means it starts cold and, crucially, needs steady volume to stay warm: Amazon SES recommends sending around 1,000 messages a day to each major provider to hold a dedicated IP's reputation, and SendGrid places its lower-tier customers on shared pools precisely because they do not send enough to justify one. Unless you send a large, consistent volume, stay on a shared IP and warm the domain only — a dedicated IP you cannot keep busy decays faster than it builds.
Use a subdomain to protect your root domain
Send marketing from a dedicated subdomain — say news.example.com or mail.example.com — and keep your root domain for the transactional mail that must always arrive. Mailbox providers treat a subdomain as its own sending identity for reputation, so a rough marketing week, a complaint spike, or a warmup misstep on the subdomain will not drag down the password resets and receipts leaving your root. The subdomain still starts from scratch — inbox providers treat it as a new, unknown sender with no reputation of its own — so it needs its own warmup and its own SPF, DKIM, and DMARC records. This is the domain-level version of keeping transactional and marketing streams apart.
Re-warming: switching platforms or waking a cold domain
Warmup is not only for brand-new domains. Any time your sending identity changes or goes quiet, you re-warm. Switching email platforms usually moves you to a new IP or pool with no history, so you ramp again even on an old domain. A domain that has not mailed in months has let its reputation lapse and needs a gentler restart. And the fastest way to burn a warmup is to point it at the wrong audience: cold outreach and purchased lists generate the complaints and bounces that destroy reputation faster than any ramp can build it. Warm with mail people asked for, never with a cold blast.
Signs your warmup is going wrong
- More of your mail is landing in spam — check seed tests and, if you have it, Google Postmaster Tools before raising volume again. Confirm you authenticated the domain first, because a warmup on an unauthenticated domain stalls on its own.
- Rising 4xx responses — soft bounces, deferrals, and throttling notices mean a provider is telling you to slow down; hold volume until they clear.
- A climbing complaint rate — if spam reports tick up, you are sending too much, too fast, or to the wrong people; pause and prune the list.
- Bounce spikes — a jump in hard bounces signals list-quality problems that warmup will amplify, not fix.
- Any of these means slow the ramp or pause it, not push through. When the signals settle, resume from a lower step — and if warmup keeps failing, revisit the fundamentals in the deliverability overview.