Do You Need a Warmup Service?
Outsourcing warmup rarely works as advertised. Here's why you're better off managing it on your own.
Warming up a domain, the process of gradually establishing trust with inbox providers, is important for consistently landing in the inbox.
Enter the warmup service.
If you are sending from a new domain or ramping up email volume, you may encounter companies promising a warmup with guaranteed deliverability improvements by simulating positive engagement.
You may be tempted to outsource warmup to a service. Our customers often ask us questions about these services.
The short answer is, "No. Warmup is important, but it's naturally achieved when you send wisely to people who want your messages."
Let's dive deeper into why warmup is important and how it works.
Warming up your domain is important
The truth is, warming up your domain is key for any new domain or any domain beginning to send at scale. While warmup is a real concept, outsourcing it rarely works as advertised.
Trust is earned through consistent sending behavior and engagement from real recipients. Inbox providers view key signals as you scale your sending volume to determine your trustworthiness as a sender.
They look at volume patterns, spam complaints, unsubscribe rates, and how real users interact with messages.
When these signals look positive and consistent over time, the domain's reputation improves or stabilizes at a high level.
We offer an exhaustive guide on how to warm up your domain and/or IP, and our team stands by ready to help create a tailored plan to meet your needs.
How to Warm Up a New Domain
Our recent experience preparing a new domain to send thousands of emails to inboxes.
resend.com/blog/how-to-warm-up-a-new-domain
How Warmup Services Work
Warmup services typically promise improved deliverability through controlled engagement. They control the sending accounts and the receiving inboxes, so they can guarantee that your emails receive opens, replies, and positive interaction signals.
They make claims such as:
We can improve your email deliverability and help you reach the inbox of your prospects. Every single time!
We mirror your real campaigns and sequences in our warmup—turning inbox rate into a strength, not a liability.
Our completely automated network tool keeps your emails out of spam and improves your sending reputation with positive email interactions.
They mimic real engagement signals and position this as proof that your inbox reputation is improving.
The problem is that these signals are not treated as meaningful by inbox providers. Although they may appear positive on the surface, the signals are not meaningful indicators of user intent. They are synthetic signals designed to imitate engagement rather than reflect actual usage.
Worse still, email service providers detect these generated patterns. Gmail, Outlook, and similar systems can determine when engagement exists only inside a closed loop of artificially connected inboxes or when there are unnatural reply patterns and repeated communication between non-organic account clusters.
Artificial engagement behavior like this can actually work against you and can eventually damage your reputation.
Real Signals vs Fake Signals
Inbox providers evaluate signals over time.
Real signals come from people who want the message and interact with it deliberately. Fake signals come from controlled accounts that are programmed to behave positively.
| Real Signals | Fake Signals |
|---|---|
| Recipients open emails they requested | Accounts open emails automatically |
| Replies occur because the message matters | Auto-replies generated from a script |
| Unsubscribes and complaints happen naturally | No complaints because no real users are involved |
| Gradual, consistent volume increases | Sudden short-term spikes in activity |
| Real user engagement continues beyond warmup | Engagement collapses when automation stops |
Inbox providers evaluate these patterns at scale. Synthetic engagement creates consistent, repetitive footprints, which are easier to detect.
Real-world data always looks varied and unpredictable.
Another risk is that automated inbox pools may contain accounts that have been reclaimed by providers. When that occurs, sending to these addresses is indistinguishable from sending to spam traps and is one of the fastest ways to damage your domain's reputation.
Why These Signals Do Not Work Long-Term
Warmup services cannot simulate the actual signals that inbox algorithms reward. They cannot generate organic opens, real customer-driven replies, interest over time, or authentic click behaviors. They also do not create consistent engagement after the warmup period ends.
Inbox providers evaluate reputation using longitudinal metrics. If activity suddenly drops once automated warmup stops, that decline becomes a negative signal. It tells the provider that engagement was never real.
These systems also look for repeated communication patterns within identical sender-recipient clusters. When your reputation is tied only to those patterns, it becomes unstable and risky. Once detected, filtering becomes stricter rather than more permissive.
Warmup services do not improve reputation. They temporarily mask the absence of real engagement and often decrease reputation once the artificial signals end.
What Actually Improves Deliverability
Good deliverability comes from:
- Predictable sending behavior
- Relevant content
- Real-world engagement
When recipients expect communication, inbox providers reward that with better inbox placement.
Reliable factors include:
- Authentication configured correctly (SPF, DKIM, DMARC—Resend requires and enables this for your domains)
- Consistent sending volume that increases gradually
- Real recipients who open and read messages
- Low bounce, unsubscribe, and complaint rates
- Content that matches user intent and expectation
Warmup does not require third-party intervention. If you follow best practices and send to people who want your messages, their genuine engagement will build your domain's reputation over time.
Some practical steps to signal inbox providers that you are a legitimate sender:
- Get proper consent to send users emails.
- Use a double opt-in form to confirm that users want to receive emails from you.
- Send confirmation emails when users sign up to a mailing list.
- Send welcome emails when users create an account.
When confirmation emails are in response to user action, they are more likely to be opened and read, which is a good signal for inbox providers.
How to Get Email Consent
What it means to get email consent and how to ask for it properly.
resend.com/blog/how-to-properly-get-email-consent
So Do You Need a Warmup Service?
Legitimate senders do not need a warmup service.
If the goal of your email program is to send legitimate messages to users who opted in, you do not need a warmup service.
Everything inbox providers care about happens naturally when your audience wants the message. Your domain warms itself through real interaction.
False engagement is not a reputation strategy.
Unless your goal is spamming, there is no reason to have anyone else warm up your domain for you.
If you rely on automation to simulate interest, inbox systems eventually detect that pattern. When that happens, filtering tightens, reputation declines, and the domain becomes harder to recover.
Sending gradually to real recipients, using good sending practices, is the only reliable way to warm up your domain. Faking trusted signals is not a shortcut and carries the inherent risk of damaging your reputation.
If you are not sure how to warm up your domain, we offer an exhaustive guide on how to warm up your domain and/or IP. If you need additional help, our team stands by ready to help craft a plan to fit your needs.