To a mailbox provider, an IP address or domain that has never sent email is a stranger. It has no positive or negative track record, and filters react cautiously: they accept a few messages, slow down the rest and sometimes put them in spam while they work things out. If a stranger shows up on day one with a hundred thousand messages, caution turns into suspicion. Warming up is the process of gradually building a credible history.
When it is needed
Warm-up is required in several situations, not only when you activate a dedicated IP:
- moving from a shared IP to a dedicated one;
- using a new domain or subdomain for sending;
- migrating to a new platform with different IPs;
- resuming sending after many months of inactivity;
- a very large jump over usual volumes, for example for a seasonal campaign.
If you use a shared pool on a reliable platform with modest volumes, you usually don't need to warm up the IP, because the address's reputation is already established. A new domain, however, should still be introduced gradually: its reputation is its own and is not inherited.
The principle: grow only when signals are good
Warm-up is not a table to follow blindly. It is a loop: send a volume, watch the results, increase only if they are good. Providers judge each sender separately, so the same IP may already be fine for one provider and still under observation at another.
An indicative plan, to be adapted, might look like this:
| Day | Indicative daily volume |
|---|---|
| 1-2 | a few hundred |
| 3-4 | roughly double |
| 5-7 | a few thousand |
| second week | 30-50% increase every day or two, if signals stay good |
| third and fourth weeks | approaching normal volume |
The numbers vary a lot depending on the target volume. If you need to reach a few tens of thousands of messages a day, three or four weeks is a realistic horizon; much higher volumes need more time.
Who receives first
Choosing the audience in the first weeks matters more than the exact numbers. Start with the most engaged contacts: people who opened or clicked recently, active customers, subscribers from the last few months. They generate the positive signals you need: messages opened, no complaints, no bounces.
The typical mistakes are the opposite:
- starting with the full list, including people inactive for years;
- using the warm-up to «clean» an old imported list;
- warming up with uninteresting messages just to build volume.
If you have transactional flows, such as order confirmations or account notifications, they are excellent candidates for the first days: they are expected and they get opened.
Spreading across providers
It helps to look at the list's composition by provider. If half the addresses belong to a single service, that service will decide much of the outcome. Many platforms let you limit sending speed per destination domain, so you can grow more slowly exactly where the list is most concentrated.
Signals to check every day
During warm-up, metrics should be read daily and split by provider:
- temporary rejections with 4xx codes mentioning limits or reputation: the provider is asking you to slow down;
- permanent bounce rate: if it rises, the chosen segment is too old;
- spam complaints: they must stay close to zero;
- engagement per provider: a collapse in opens at one service often means messages are going to spam there;
- data from sender tools, such as the spam rate shown in providers' dashboards.
When a signal worsens, the right response is to stop increasing or go back to the previous day's volume, not to push on. Forcing a provider that is throttling you usually leads to longer blocks.
After the warm-up
The reputation you build is not permanent. An IP that sits idle for weeks cools down, and irregular volumes, with long pauses followed by spikes, are in themselves an unreassuring signal. That is why senders with highly seasonal traffic often do better on a shared pool, or by keeping a small, steady flow even in quiet periods.
In my work, the most common cause of a failed warm-up is not an overly fast plan but the wrong list: a good schedule cannot make up for contacts who don't want the messages.
In short
- An IP or domain with no history is treated with caution and must be introduced gradually.
- Volume grows only when signals, read provider by provider, stay good.
- The first weeks should go to the most engaged contacts and to transactional flows.
- Temporary rejections and rising complaints tell you to stop or slow down.
- Reputation cools with inactivity: steady volumes keep it alive.
