Warm-up advice gets muddled because teams often change more than one thing at once.
They launch a new brand domain, move to a new ESP, add a new dedicated IP, change the bounce domain, and switch message format in the same week. Then deliverability drops and the postmortem says, "the domain was not warmed up" even when the real issue was the IP, or the recipient quality, or broken alignment.
The practical answer is simpler than the folklore:
Warm up the thing that is new to the receiver and is carrying meaningful reputation signals.
In practice:
That is the operational frame that avoids most confusion.
Mailbox providers do not score one single "sender reputation" number.
They look at multiple layers at once, including:
From domainGoogle is explicit that Gmail tracks volume, feedback, and limits per domain and IP address, and that DKIM and SPF quotas are domain-specific while IP quota is shared across senders on that IP. It also recommends slow, consistent increases and separate ramping after significant infrastructure or header changes. Source: Google's Email sender guidelines.
Microsoft is equally explicit that Outlook.com filtering is influenced by the sending IP, domain, authentication, list accuracy, complaint rates, content, and more. Source: Outlook.com's Postmaster troubleshooting guidance.
So the right question is not "do domains warm up or do IPs warm up?"
The right question is "which sender identity just changed, and which reputation layer will the receiver have to trust from scratch or near-scratch?"
This is the easiest case.
Yes, a new dedicated IP needs warm-up.
Amazon SES says providers are less likely to accept mail from new IP addresses with little or no history, and recommends gradually increasing volume while sending to your most active users first. Source: Amazon SES Warming up dedicated IP addresses.
Microsoft says new IPs typically do not have reputation yet, are more likely to experience deliverability issues, and may need a couple of weeks or sooner to ramp depending on volume, list accuracy, and complaint rate. Microsoft also notes that a new IP under an already-authenticated domain can inherit some of that domain's reputation. Source: Outlook.com's Postmaster troubleshooting guidance.
That last point matters.
A strong established domain can help a new IP ramp faster, but it does not make the IP instantly warm. It just means the cold-IP problem may be less severe than if both the domain and IP are brand new.
If this is your situation, IP address migration for mail goes deeper on the overlap strategy.
If you start sending serious volume from a domain that has little or no sending history with major mailbox providers, treat that like a warm-up case even if the infrastructure is technically fine.
Google's FAQ says enforcement for new domains is on an accelerated timetable, and defines a new domain there as one that has not sent more than 5,000 emails a day to personal Gmail accounts since January 1, 2024. Source: Google's Email sender guidelines FAQ.
That does not mean every small new domain is automatically blocked. It means you should not assume a fresh visible identity can be pushed immediately at full campaign volume just because SPF, DKIM, and DMARC are present.
What usually needs ramping here is not only the SMTP source. It is the trust story around the domain itself:
Authentication is still mandatory. But authentication alone is not the same thing as history.
This is the case that causes the most bad advice.
Some teams hear that subdomains inherit from the parent and conclude there is no warm-up at all. Others treat every new subdomain as if it were a completely unrelated new company. Neither view is quite right.
Google's FAQ says bulk sender classification counts all messages sent from the same primary domain. Its example says mail from solarmora.com and promotions.solarmora.com counts together toward the 5,000/day threshold. Source: Google's Email sender guidelines FAQ.
That means subdomains are not invisible to the reputation and compliance story of the parent brand.
At the same time, a new subdomain is often a new stream with its own:
So what is the practical answer?
If news.example.com starts sending promotional mail for the first time, do not assume the mailbox provider sees it as fully equivalent to long-stable receipts from example.com or notify.example.com.
The parent brand history can help. The exact subdomain is still a new behavior pattern.
That is why dedicated subdomains are useful operationally but are not magic reputation bypasses. They help you isolate streams, which is good. They do not remove the need to ramp volume sensibly.
A new subdomain is usually a partial warm-up problem, not a zero warm-up problem and not always a full-from-scratch reset. Treat it as a new sender stream under a related brand identity.
If your main goal is clean stream separation, Transactional vs marketing email separation is the best companion post.
Not every mail change is a warm-up event.
Usually, these changes do not create a separate warm-up requirement on their own if the visible sender, recipient quality, and overall traffic pattern remain stable:
p=none to enforcementThose can still break deliverability if implemented badly. They just are not reputation warm-up events in the same sense as a fresh IP, fresh domain, or brand-new stream.
In real life, warm-up risk comes from combinations.
This is the hardest launch shape.
Both the visible identity and the infrastructure are cold. Be conservative.
Mostly an IP warm-up case, but domain reputation can soften the ramp if the domain is already trusted and authentication stays aligned.
Usually more of a stream and domain-introduction problem than a raw IP problem, because the IP pool may already be established while your exact stream is new.
Still a warm-up risk, even if no DNS names changed.
Google specifically recommends gradually increasing the modified segment of traffic after significant infrastructure or header changes. That same caution makes sense when the audience or content profile changes sharply too. Source: Google's Email sender guidelines.
Teams often say they are warming up a domain when what they are really doing is this:
That is not a single warm-up variable. That is five.
When that bundle fails, the root cause gets blamed on whichever noun was most recently discussed in a meeting.
If you want a cleaner diagnosis, separate these questions:
That breakdown gets much closer to what mailbox providers actually evaluate.
DMARC does not warm up anything by itself.
What it does is remove ambiguity about which domain is claiming responsibility for the message. Google requires direct mail for bulk senders to align the From domain with SPF or DKIM, and Microsoft documents that SPF, DKIM, and DMARC are only part of a broader trust evaluation. Sources: Google's Email sender guidelines and Microsoft's Email authentication in Defender for Office 365.
That means DMARC helps warm-up indirectly because it gives receivers a stable aligned identity to build history around.
If you are ramping a domain or subdomain, DMARC and sender domain warm-up covers that interaction in more detail.
Use this rule:
That rule is not perfect, but it is much better than arguing that only domains matter or only IPs matter.
If there is one thing worth keeping boring, it is the launch pattern.
Google recommends a consistent rate, engaged users first, and slower sending if bounces or deferrals rise. Outlook.com points to complaint rate, list accuracy, domain, and IP reputation as key factors in deliverability. Those two documents together are enough to justify a cautious rollout even before looking at any vendor-specific playbook.
New IPs need warm-up most obviously.
New domains also need cautious ramping because authentication is not the same thing as sender history.
New subdomains sit in the middle: they benefit from the parent domain context, but they are still new sender streams and should not be launched recklessly.
So the safest answer is not "warm up the domain" or "warm up the IP" as if only one can be true.
Warm up whichever trust layer is new, and avoid changing too many trust layers at the same time.