This page is part of the Customer.io documentation. For the complete documentation index, see llms.txt.

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Safely warm up your sending domain

To ensure your emails successfully reach your recipients, it’s important to build a good reputation for the domain you’re sending emails from, which is where a domain warm-up process comes in.

Overview

Think of warming up your domain like introducing yourself to inbox providers (like Gmail, Yahoo, and Microsoft) before you start sending at full volume. When a new or unfamiliar mail stream appears, inbox providers don’t know whether it’s legitimate, or whether someone is attempting to impersonate a brand or hijack a domain’s existing reputation. By gradually increasing volume from a controlled starting point, you give providers the opportunity to observe consistent sending behavior, verify that you are who you say you are, and confirm that your recipients are actually engaging with your messages. This is how providers determine that a new mail stream is a legitimate ramp and not an attack.

Newly registered domains

If your domain was registered within the last 30 days, exercise extra caution. Many inbox providers flag mail from very recently registered domains as inherently high-risk, regardless of content or authentication. This is especially true for Gmail and Microsoft. If possible, delay the start of your warm-up until the domain is at least 30 days old. If you must send sooner, start at significantly lower volumes than the schedule below and expect slower progress.

Third-party domain warming services

A growing number of vendors offer domain warming services that claim to build sending reputation on your behalf before you begin sending to your real recipients. These products are marketed under various names—including deliverability tools, inbox placement services, and engagement networks—but they generally work the same way: by sending mail to a network of mailboxes they control and simulating opens and clicks to create the appearance of engagement.

Major inbox providers, including Gmail, Yahoo, and Microsoft, are well aware of how these services operate. They can and do detect the traffic patterns they produce, and they do not extend any reputation credit for it. The synthetic engagement carries no weight in their filtering decisions.

There is also a compounding risk. If you’ve used a domain warming service on a domain and then switch to sending to your real recipients, providers see an abrupt shift from artificial to real traffic patterns on the same domain. The domain doesn’t carry any of the “reputation” the service claimed to build, and depending on how aggressively it was used, the domain may be in a worse position than if you’d started fresh.

If you’ve previously used a domain warming service on a domain you intend to use with Customer.io, treat that domain as cold and warm it from scratch using real recipient traffic only.

Sending domains in Customer.io

When you set up email sending in Customer.io, you send from a Customer.io-managed subdomain (for example, cio123456.yourdomain.com). This subdomain is always new from the perspective of inbox providers, even if your root domain has years of sending history. Use the warm-up schedule below for all new sending setups, regardless of whether your root domain is established.

Warm-up schedule

Each stage lasts 2–3 days. This is intentional: inbox provider reputation signals, including Google Postmaster Tools, typically lag 1–2 days behind actual sending. Holding at each volume level for 2–3 days gives providers time to process your traffic before you increase it.

Volume increases should not exceed 1.5x the previous stage. Recent trends from inbox providers indicate that doubling volume at each step is too aggressive. Stick to 1.5x or less. If negative signals appear at any stage, hold, or reduce volume rather than continuing to advance.

These schedules are guidelines. The right pace for your warm-up depends on your engagement rates, list hygiene, and how inbox providers are responding to your mail. Let the data guide you.

Domain warm-up schedule

Key notes:

• Never exceed 1.5x the previous stage’s daily volume in a single day.
• While daily volume is the more critical limit, throttling your hourly send rate is strongly recommended. Sending your full daily volume in a short burst looks very different to inbox providers than the same volume spread evenly across the day. Aim to distribute sends as consistently as possible throughout your sending window.
• You only need to warm your domain to your actual regular sending volume. If you typically send broadcasts to 30,000 subscribers, warm to 30,000 and don’t continue beyond that unnecessarily.

Each stage is listed as 2–3 days. Use 3 days as your default. Reputation signals at inbox providers, including Google Postmaster Tools, can take 1–2 days to reflect recent sending activity, so staying at each stage for at least 3 days ensures you make advancement decisions based on stable data. Two days is acceptable when all monitored signals have been consistently healthy throughout the stage, but err toward 3 days in the early stages and any time metrics show even minor softness.

Core best practices

1. Start with your most engaged subscribers. Front-load early warm-up cohorts with subscribers who have opened emails within the last 90 days. Contacts who haven’t engaged in 120 days or more should be held back until the domain is fully warmed, and then gradually reintegrated into the audience.

2. Target engaged recipients throughout. Engagement is the primary signal providers are evaluating. High open rates early in the warm-up carry more weight than volume. Do not sacrifice list quality to hit volume targets faster.

3. Don’t switch domains mid-warm-up. Starting warm-up on one domain and switching to another mid-process resets all progress. Commit to your sending domain before beginning, and see the warm-up through on that domain.

4. Warm-up is ongoing. Completing the warm-up schedule doesn’t mean you’re permanently warmed at that volume. Any future volume increase greater than 1.5x your established daily average should be treated as a new warm-up event and ramped accordingly. This applies to one-time events like peak season campaigns just as much as permanent volume growth.

5. Use Google Postmaster Tools while it’s available. Set up Google Postmaster Tools before you start warm-up so you have visibility into your Gmail reputation and compliance throughout the process. You need to set up the Customer.io-specific subdomain in Google Postmaster Tools to see the most relevant data.

    Google has signaled that the domain reputation feature in Postmaster Tools v1 will be retired in the future, though it can still provide a useful warm-up signal in the meantime.

   Ensure your Email Compliance Dashboard in Google Postmaster Tools v2 shows all items as compliant before beginning warm-up, and treat any change in compliance status as a reason to pause and investigate.

Monitor during warm-up

After each stage, review the following before advancing:

• Gmail domain reputation (Google Postmaster Tools, while available): Should remain at Medium or above. A drop to Low or Bad means stop immediately.
• Spam rate (Google Postmaster Tools): Should stay below 0.08%. At 0.10% or above, pause and investigate before advancing.
• Bounce rate: Bounce rates above 2% indicate list hygiene problems that you should resolve before advancing. During warm-up, rates approaching 2% warrant investigation even if you haven’t crossed that threshold, since warm-up cohorts should be your cleanest lists.
• Throttling and deferrals: Some temporary deferrals from major providers in the early stages are normal. Persistent deferrals, especially at Gmail or Microsoft, warrant a pause.
• Email Compliance Dashboard: Any compliance flag should be treated as a hard stop.

Broadcasts vs. campaigns

• Broadcasts: Easier to troubleshoot if issues arise. Ideal for most warming scenarios. Broadcasts support daily ramp, which can automate your warming schedule without requiring you to create a new broadcast for each stage. When configuring daily ramp, set your starting volume, target volume, and number of days to match your warming plan. Set the ramp period long enough to follow the 1.5x progression in the preceding schedule—up to 60 days is supported.
• Campaigns: Some senders use campaigns with low volumes, starting a new one every few days during the warming period to align with the ramp schedule. Either method works as long as you follow the plan.

For additional guidance on maintaining deliverability after warm-up, see Email Deliverability Best Practices.