How to warm up a sender domain - Online Help | Zoho Campaigns

How to warm up a sender domain

In short: warming up a sender domain means gradually increasing your email volume—starting with your most engaged contacts—so mailbox providers learn to trust you. Authenticate first (SPF, DKIM, DMARC), ramp over about four to six weeks while watching your spam-complaint and bounce rates, and pause if deliverability slips. It can’t be rushed.
We know that as marketers, you want to send large volumes of emails and achieve inbox placement. When mailbox providers receive emails, they consider the reputation of a sender domain to make inbox decisions. So how is that reputation calculated? Mailbox providers use different techniques to grade a sender domain. Some assign a score (0–100), while others rank reputation as “low,” “medium,” or “high.” Here are the factors they consider:
  • Email volume
  • Email engagement (opens, and more reliably, clicks)
  • Spam markings and classification
  • Spam trap hits
  • Bounce rate
  • Total complaints received
  • Unsubscribe rate
Before sending an email campaign, make sure you’re using a sender domain with a positive reputation. Otherwise, your emails may face deliverability issues. If you decide to increase your email volume or use a new sender domain, warm the domain up first. Warming up is the process of earning a positive reputation from mailbox providers. It can’t be done overnight—it’s a methodical process that typically takes about four to six weeks (a couple of weeks is the bare minimum for lower volumes).

Warming up a sender domain – An infographic

Let’s dig in and learn how to successfully warm up your sender domain.

Warmup prerequisites

Complete these before you send a single warmup email—authentication in particular is now a hard requirement, not a nicety. Since February 2024, Google and Yahoo (and Microsoft from May 2025) require any sender of 5,000+ messages a day to authenticate; as of November 2025 Gmail applies temporary and permanent rejections to non-compliant mail. Don’t begin warmup without the prerequisites in place.
  • Authenticate your domain with SPF, DKIM, and DMARC, and keep your records and domain keys up to date. Start DMARC at p=none and tighten toward quarantine or reject as you gain confidence. See how to authenticate your sender domain. Also add one-click unsubscribe to your marketing mail.
  • Verify the WHOIS record for the sender domain so ownership details are complete and consistent.
  • Set up Google Postmaster Tools to monitor the reputation your domain earns at Gmail. This is non-optional: it’s the clearest real-time signal of how Gmail grades you, and it’s where you’ll watch your all-important spam-complaint rate.
  • Clean your mailing list and build a segment of active contacts. Suppress hard bounces immediately, drop addresses that soft-bounce repeatedly, and exclude anyone with no opens or clicks in the last ~90 days. This engaged segment is the list you’ll mail during warmup.

Warming up your sender domain

After you complete the prerequisites, you can begin warming up. Warming up gradually increases your email volume; as contacts engage with your emails, your domain earns a positive reputation. By the end, your domain will be primed to send the volumes you have planned. Two principles matter more than any exact number: increase volume gradually (roughly 20–40% per step, never doubling into a flooded inbox), and keep sending at a steady, predictable daily rhythm rather than in bursts.

The table below is a sample ramp. Treat it as a guide, not a rule—let your engagement and your Postmaster Tools signals set the pace. If complaints climb toward 0.3% or bounces approach 2%, slow down or hold volume steady before continuing:

Day
Number of emails
1
50
2
50
3
100
4
100
5
200
6
200
7
400
8
400
9
800
10
800
11
1,600
12
1,600
13
3,200
14
3,200
15
6,400
16
6,400
17
12,800
18
12,800
19
25,600
20
25,600
21
51,200
 
Start by sending to active contacts first—they’ll engage and help your domain earn a positive reputation. For the first two weeks, mail engaged contacts only. From the third week, you can slowly begin including the rest of your contacts.

Handling deliverability issues

If you face deliverability issues during warmup, pause sending, identify the issue, and take corrective measures. Here are common issues and how to resolve them:

When your email gets classified as spam

When mailbox providers or contacts mark your emails as spam, they negatively impact your domain’s reputation. These tips help prevent it:

Subject line

    ● Don’t capitalize the entire subject line
    ● Don’t overuse punctuation marks
    ● Write naturally rather than chasing a list of “banned” words—modern filters weigh sender reputation and engagement far more than any single term
    ● Don’t use fake Re/Fwd tags

Content and phrasing

Older guidance leaned on avoiding specific “spam trigger words.” In practice, today’s filters rarely block on individual words alone; they judge the whole picture—your authentication, reputation, engagement, and how closely the content matches what recipients expect. Still, avoid the patterns spammers overuse: exaggerated money or “get rich” claims, deceptive urgency, spaced-out letters (like F R E E), and mismatched or misleading subject lines. Write clearly, deliver what the subject promises, and keep a healthy text-to-image balance.

URLs

Don’t use shortened URLs, blocklisted URLs, or URLs that point to blocklisted domains. At Zoho Campaigns, we recommend sending your campaign to test accounts before sending to your contacts—this lets you verify the content and confirm every URL works correctly.

When the sender domain earns a negative reputation

If your emails hit spam traps, or contain blocklisted URLs or URLs of blocklisted domains, your sender domain will earn a negative reputation. (“Blocklist” is the current term for what used to be called a “blacklist.”) In these situations, pause the warmup, identify the issue, and take corrective measures. Wait to resume until the issue is fully resolved—and note that even after you’re delisted, mailbox providers’ own reputation systems can keep treating the domain cautiously for two to four weeks, so resume gently.

Temporary email deferral

A temporary deferral means the receiving server declines to deliver your emails for the time being (you’ll often see a 4xx response such as “421”). Your emails won’t be delivered until the cause is resolved. If your mail is deferred due to a high sending rate during warmup, reduce your volume: send the next batch at about half of your last send, then slowly increase again after successful delivery. Don’t panic-resend the same batch repeatedly, which only compounds the problem.

Resuming the warm-up process after a pause

Say you were sending 25,000 emails per day, then stopped due to deliverability issues. If you resume weeks later by sending 25,000 again, you’ll hit delivery problems. Mailbox providers track your domain’s sending history and reputation, and a domain that has been quiet for around 30 days effectively decays and needs re-warming. A sudden spike after inactivity looks suspicious and leads to rejections. So restart low—around 50 emails a day to your most engaged contacts—and rebuild volume gradually, just as you did the first time.

Best practices to warm up a sender domain

Throughout warmup, monitor the campaigns you send and keep tabs on the metrics that matter—prioritizing the reliable ones:
    ● Lead with click rate, bounce rate, and your Postmaster Tools spam-complaint rate. Since Apple’s Mail Privacy Protection inflates open rates, treat opens as a soft signal and lean on clicks and complaints instead.
    ● Check your domain against blocklists periodically to confirm it hasn’t been listed, and monitor domain reputation weekly.
    ● Adopt hygienic sending practices—honor contacts’ time zones, use targeted sending, and re-verify aging lists so they don’t decay into spam traps.
    ● Model your best past campaigns to maximize engagement, and use A/B testing (decided on clicks, not opens) to identify your strongest subject lines.
    ● Consider a dedicated sending subdomain (for example, email.yourdomain.com) so that if a marketing issue arises, it’s contained and your primary domain’s reputation is protected.
Warming up a sender domain isn’t something you can accomplish in a couple of days—it usually takes several weeks. The best way to do it is by adopting hygienic email-sending practices and increasing your volume gradually and predictably.