You set up the technical authentication records correctly, your mail domain is ready, and yet the first messages you send from a new address land in spam at a rate that makes no sense. It is not a configuration mistake: a freshly created mail domain has no reputation at all yet, and mail providers worldwide are conservative by default with any sender they do not recognise.
Why a new domain starts at a disadvantage
Mail services (both yours and each recipient's) constantly assess the reputation of every domain sending them messages: how long it has existed, what volume it typically sends, what share of its messages get marked as spam, how many bounce. A domain with years of history and good behaviour has room for trust. One that appeared yesterday has no data in its favour, so the system defaults to treating it with suspicion.
If that new domain also starts sending a high volume from day one, the behaviour looks too much like the typical pattern of a spammer just standing up infrastructure, which worsens the initial suspicion instead of easing it.
The logic behind warm-up
Warming up a mail domain means starting with low volume, to recipients likely to open the message and not mark it as spam, and gradually increasing over several weeks. It is, essentially, building a reputation bit by bit, instead of trying to prove it all at once with a mass send from day one.
The principle behind this: spam detection systems pay close attention to sudden behaviour changes. A sudden jump from zero to thousands of daily sends is exactly the pattern they look for to slow down, even if the content of those messages is perfectly legitimate.
How to do it in practice
Start with low volume and quality recipients. In the first weeks, send few messages per day, and prioritise addresses where you know the message will be well received: contacts who already know you, active customers, people who specifically asked to receive that mail. Avoid cold or purchased lists at this early stage.
Increase volume gradually. Each week, if behaviour has been good (few bounces, good open rate, few spam marks), bump the volume up a bit. There is no exact universal formula, but the general idea is to double gradually instead of jumping straight to the final volume you need.
Take care of content quality from the very first send. A spam-sounding subject line, a message with only an image and no text, or a design with too many links triggers content filters regardless of the domain's reputation. Warm-up does not compensate for a badly built message.
Pay attention to rejection signals. A bounce (the address does not exist or rejects the message) or a spam mark are negative signals that affect the reputation being built. Keep sending lists clean, removing addresses that bounce consistently.
Send some mail, not none, throughout the process. A domain that sends very little for a long stretch also fails to build a history. Warm-up requires sustained activity, even if low at first, not total silence with occasional spikes.
How long it takes
There is no fixed number that applies to every case, because it depends on the final volume that needs to be reached and how careful the process is. As a general reference, a reasonable warm-up process usually stretches over several weeks, not days, before reaching the sending volume an established domain can sustain without issue.
Rushing the process almost always costs more than going slow: a domain flagged as suspicious for moving too fast can take much longer to regain mail providers' trust than one that took the process patiently from the start.
Mistakes that ruin warm-up
Buying an address list and sending it mass mail from the new domain. It is the fastest way to ruin reputation before having built any.
Ignoring bounce and spam-mark rates during the process, instead of adjusting volume or cleaning the list when negative signals appear.
Jumping straight to the final volume as soon as the domain is set up, with no prior warm-up period.
Switching mail domains frequently, which resets the warm-up process every time, instead of keeping one stable long-term and protecting its reputation over time.
What to do if the domain already has a bad reputation
If you inherited a domain with a sending history, or if a domain of your own has already started landing in spam repeatedly, the process is not exactly the same as for a new domain, but it looks similar: volume needs to drop sharply, the sending list needs cleaning of problem addresses, and reputation needs rebuilding with the same gradual care, instead of pushing on with the same volume that caused the problem.
Recovering a damaged reputation usually takes longer than building one from scratch, so it is worth acting as soon as the first negative signals appear, rather than waiting until the problem becomes obvious to every recipient. Support at your hosting or mail provider can often confirm whether a domain is currently flagged, which is a faster starting point than guessing from delivery symptoms alone.