How to Warm Up an Email for Cold Outreach

How to Warm Up an Email for Cold Outreach

Email servers tend to be very picky when deciding on which emails to deliver.

They’re not mean by any chance. They’re just flooded with millions of spam emails every single day and would rather filter out a few legitimate emails here and there than risk one phishing email landing right into someone’s inbox.

This pickiness can be particularly annoying for people looking to get started with sending cold email campaigns. If you’re one of them, you probably haven’t sent many emails yet from your domain. Because of that, email servers will be particularly cautious when accepting your first campaigns.

What can you do about it? Firstly, once you’ve found the email addresses of potential leads, you should verify them. But that's just the first step.

You also need to do a proper IP warmup and warm up your email address. This is the process of teaching email servers that you’re a legitimate sender and that they should treat your emails accordingly (meaning, deliver them right into the recipient’s inbox).

There are various techniques for doing this, both manual and automated. In this blog post, I'm going to cover both approaches.

1. Email deliverability in a nutshell

Email deliverability is the art of getting emails delivered right into recipients’ inboxes. And not just anywhere in their inbox – you don’t want your emails lingering in someone’s “Promotions” folder, never to be opened.

The aim of any effort put into deliverability should be to have every email land in the “Primary” folder of the recipient’s inbox. No exceptions.

Only that kind of inbox placement guarantees that most people will see your message. If they see it, they may just open it and even take the desired action.

On the other hand, if they never come across your email, they won’t convert, no matter how hard you try.

If you automate cold outreach and send hundreds or thousands of emails every day with Hunter Campaigns or any other tool, this can hurt you badly when you send emails from a new domain. For example, let’s say that for 1,000 emails sent without any effort on the deliverability front, 800 were delivered (some to to the primary inbox but most to ‘Promotions’).

For example, only 15% (120 emails) were opened. Out of those, 20% (24) replied, and 10% (~2) of those eventually converted and bought from you a service with a lifetime value of $100. The profit from the campaign is $200.

Now, imagine that you worked on deliverability and can improve both the percentage of emails delivered and their inbox placement.

Continuing the example, 900 emails were delivered (up from 800 before), with a 22% (198 emails) open rate. The rest is the same — 20% (~40) replied, 10% (~4) of them converted, each with the same LTV of $100. Profit from the campaign just doubled.

Of course, I can’t guarantee that by doing a few deliverability tweaks, you’ll immediately double your revenue. But over time, you should be able to see a visible boost in results by implementing best practices.

Main factors impacting email deliverability

The algorithms analyzing emails are very complex. They look at dozens of factors when assessing emails. Although we don’t know how they work exactly, years of research provide some pretty clear answers on what matters the most:

1) Sender reputation (also referred to as ‘email reputation’ or ‘domain reputation’) – You build up your sender reputation by sending quality emails that don’t bounce or end up in the spam folder. Recipients also don’t mark them as spam, and they engage with them, replying or clicking on links.

When it becomes a typical pattern, email service providers (ESPs) start to assume that you’re okay after all and begin to treat your emails with more kindness.

Building a solid reputation when you’re new to cold emailing may seem like a challenge. That will be the purpose of our IP warmup efforts.

2) Email authentication – To fight spam and phishing, some serious people in the email world came up with authentication methods such as SPF, DKIM, and DMARC. You can think of these as digital certificates with which you tell an email server, “Hey, I’m Kate; here’s my ID. I’m carrying this message with me, and I certify that it should look like this and that. Oh, and I’m the sender of this message.”

ESPs validate your email authentication efforts and, if everything checks out, are more likely to accept your email. Forget to authenticate, and they’re likely to discard your email right away.

3) Verified contact list – When sending cold emails, you don’t always know if an email address exists. This can happen when you’re trying to guess someone's email address or use a purchased email list.

I can’t emphasize enough the importance of verifying each email before sending, not by sending. Although certainly, sending an email to an email address you’re unsure of immediately grants you a reply on whether it’s a legitimate email or not.

Email sent to a non-existent address

The downside is that this report about emailing a non-existent address (it’s called a ‘hard bounce’) is also shared with ESPs. The more hard bounces you get, the worse your sending reputation becomes.

Verifying emails with Hunter is extremely simple. Simply log in to your account and head over to the Email Verifier page. Enter the email address in question, and you'll see if it's safe to email or not in seconds.

Verifying emails with Hunter

You can also verify a list of email addresses with the Bulk Email Verifier.

Bulk verification with Hunter

Keep in mind that there are many more deliverability factors that can make a difference. Here are some of them:

  • Sending with a reputable provider
  • Maintaining a consistent schedule
  • Avoiding blacklists and spam traps
  • Refraining from using spam trigger words
  • Avoiding attachments when unnecessary
  • Including a link for recipients to opt out of your emails

2. IP warmup – The basics

Going back to IP warmup, I already explained that it’s a process of establishing your reputation as a legitimate email sender.

Building an email reputation is a long process that actually never ends. It’s not like ESPs quickly check up on you when you’re getting started, give you the green light and move on to the next newbie. Quite the contrary, the more you send, the more detailed sending history they will have of you, and the more informed decisions they’ll be able to make.

Getting started is hard, though, because you have close to zero history to showcase. This is where a solid IP warmup process can do wonders. It can be done in two ways:

  • Manual IP warmup – Involves manually emailing more and more email addresses, maintaining the right schedule, and gradually increasing the volume.
  • Automated IP warmup – There are also dedicated platforms that can warm up the IP address for you. This approach requires less effort on your side.

Things to do ahead of IP warmup

Whichever approach you choose to go with, there are a few things you'll want to set up first.

1. Consider sending from an alternative domain

If you’re worried about the performance of your emails, it may be worth it to set up an alternative domain that you’ll use only for sending cold emails.

Alternative domains for cold emailing

The idea is that cold emails tend to have a higher unsubscribe rate than transactional or even marketing emails.

As a result, sending cold emails could potentially negatively affect the deliverability of your company's transactional emails or other important communication.

2. Authenticate

I've already mentioned that email authentication is an important aspect of email deliverability. There are three email authentication protocols that you'll want to set up:

  • SPF (Sender Policy FrameworK) – Tells email servers which IP addresses (domains) are authorized to send emails on your behalf.
  • DKIM (DomainKeys Identified Mail) – A digital certificate that confirms the authenticity of your message.
  • DMARC (Domain-based Message Authentication, Reporting & Conformance) combines both SPF and DKIM checks and lets you decide how to handle emails that fail either check.

We have a separate guide on implementing email authentication protocols.

3. Make sure your email doesn't read like a spam email

Words you use in an email matter. If you like to browse your spam folder every now and then, you’ll see some commonly recurring words there and phrases there, such as:

  • Great offer
  • Instant
  • Free
  • Giving away

The thing is, ESPs hate these types of phrases even more than we do, simply because they’re often associated with spam. Try to avoid these and many similar words in your emails. Here's a guide that can help.

4. Show a bit of patience

Do you know what another common pattern for spammer is? They register a domain and one or more email accounts. Then, they send out the first wave of spam on the same day. If you’ve just registered a domain, it’s worth waiting a bit until you send the first email, so you’re not flagged as a spammer.

For new domains, it’s recommended to wait a couple of months before you start sending any large-scale email campaigns.

If someone has already used your domain for sending emails and you only set up a new email account, it’s not necessary to wait that long. Instead, try to hold on for about 10-14 days and then send your first emails.

3. Manual IP warmup

At this point, you probably have the stage set up for the warmup process to begin. Let’s do it then!

First things first, find a group of at least 20-30 people that would be up for sending some emails. These could be your friends, family members, former colleagues – basically, whoever in your network wields a weapon called an email account.

Look for people who use company domains (such as rather than free email providers. The more unique domains you can find, the more credible you’ll look in the eyes of ESPs.

Even better if your recipients have email addresses registered on domains hosted by many different – Google, AWS, Yahoo, GoDaddy, Cloudflare, etc.

Tell them you’ll send them a few emails shortly, and you want them to:

  • Open the emails
  • Mark the emails as not spam if they land in the spam folder
  • Mark each message as ‘Important’
  • Reply to the emails
  • Forward the emails to someone they know

Example IP warmup schedule

Start off slowly. In the first week, I would recommend sending 15-20 unique emails every day. Try not to send them to the same recipients. So, for example, if you have a batch of 30 recipients, send to #1-15 on Day 1, #16-30 on Day 2, and send only to odd numbers on Day 3. Mix them up freely in the following days.

Couple tips:

  • Don’t send all emails at once. Instead, try to break the recipients into smaller batches and send to them at different times of the day.
  • It’s okay to skip some days (e.g., weekends). But if you do that, try to send on each weekday not to prolong the process unnecessarily.
  • It’s okay if someone doesn’t reply every now and then. As long as you maintain a healthy reply rate (30% or more), you’ll be okay. But the more replies you get, the faster you’ll have a warmed-up IP.

In the second week, increase your sending volume and try to send 30-40 emails every day. It’s okay if someone gets more than one email in a day. Try not to send them a few emails in a row without getting a response, though. Instead, wait for them to reply and then reply back.

In the third week, aim to send about 50 to 80 emails. You’ll inevitably contact the same folks more than once a day, but that's fine.

Finish up with week four and about 80 to 100 emails a day.

Manual warmup schedule

If possible, try to smooth out the transitions between the weeks. So, for example, if you send 30 emails in week 2 and plan for sending 60 in week 3, don’t just double the volume overnight. Instead, aim for sending about 45-50 towards the end of week 2 and start week 3 with 55-60 emails a day. The more consistent your schedule looks, the better.

After four weeks, your IP is ready to go. You can plan the first campaign, though try not to make too big of a splash. For example, if your week 4 was about sending 100 emails a day towards the end of the cycle, but the first campaign requires 500 emails to be sent, try the following:

  • Day 1 (after the warmup is finished): send 150 emails from the campaign
  • Day 2: send another 150-180 emails
  • Day 3: send whatever’s left

Even if your IP is warmed up, a sudden spike in volume may not be viewed favorably.

4. Automated IP warmup

If this sounds too overwhelming, an alternative approach would be to automate the warmup process. It’s useful if you don’t have much time to plan and execute the schedule, or you would rather spend it somewhere else.

Both methods can also be used in conjunction – starting off with a manual warmup and then plugging in a tool to finish up the process. There’s also nothing wrong with using an automated IP warmup tool from the get-go.

How does automatic IP warmup work?

The way these tools work is quite interesting. They set up a network of hundreds or even thousands of unique email addresses. They authenticate them and incorporate all sorts of tricks for them to be treated as real senders. Some even get their users involved in the warmup process, creating a network as reliable as it possibly gets.

Once you integrate your inbox with a warmup tool, they will start sending emails to some of these accounts, slowly at first, speeding up a bit with time. Each account will “show interest” in your emails, open them, respond with genuine replies, and interact with links.

IP warmup schedule

They would even go as far as diving into their spam folders (where some of your emails will inevitably go), marking your emails as not spam, and producing a reply for each. All of this is done to show email servers that you send valuable emails and that your audience is genuinely engaged. And it works!

Examples of automated IP warmup tools

There are plenty of options in this field. Here are my three personal favorites:

1. Warmbox

Warmbox homepage

Warmbox operates over 10 thousand accounts across the globe, closely resembling those used by real people. It builds a schedule for you but lets you customize it — you can adjust the reply rate and the number of emails sent each day.

The tool will send realistic emails, respond to emails automatically, dig them out of the spam folder, and mark each of your messages as an important one. Another helpful feature that's included is inbox placement monitoring.

2. Mailwarm

Mailwarm homepage

Mailwarm sends your warmup emails to over 1,000 accounts, generating replies and all sorts of positive interactions. Mailwarm lets you configure precise numbers if you already have an idea of how many emails you want to send each day.

If you don’t, it will suggest a warmup strategy for you.

3. Warmup Inbox

Warmup Inbox homepage

Warmup Inbox exchanges emails between your inbox and the real accounts of over 4,500 of its users.

The platform automatically interacts with your emails, boosting your deliverability in the process. It also constantly monitors for your domain's presence on blacklists and alerts you if you end up on one of these lists.

5. Email warmup never really ends

Once your IP is warmed up, you’re ready to start your outreach campaign.

I would encourage you to monitor your deliverability on a consistent basis. Sudden drops may happen, and it’s important to react right away if they do. It could be that you’ll end up on a major blacklist. A campaign may generate lots of spam reports. Or perhaps you started off too aggressively, and you need to adjust your warmup schedule.

Some useful tools for testing your deliverability are Mail-tester, GlockApps, and Sender Score.

I would highly recommend that you try to balance your campaigns and spread them out over several days or even weeks. Sending out too much at once may get you suspended by your ESP, even if your IP is really, really warm.

A helpful thing to do is also a continuous IP warmup. It’s about sending some of the warmup emails even after you’ve already started cold emailing real people.

Why? Because outreach campaigns inevitably create spikes. You will have quiet days with no emails and days with hundreds of them. What’s more, many of your emails will likely be ignored due to the nature of outreach, maybe manually put in spam, or archived. This doesn’t help your reputation much.

You can use an email warmup tool to send and engage with your emails at all times. You can also do it manually as you did before. Even 20-30 emails a day will make a difference. Let me show you an example.

Let’s say you’ve warmed up an email address and started sending cold email campaigns. Many say that Mondays and Fridays are bad for sending, and weekends are out of the question. That leaves you with three days in a week to send 600 emails. From your experience, you expect a 3% reply rate.

Example schedule and results:

  • Friday - 0 emails
  • Saturday - 0 emails
  • Sunday - 0 emails
  • Monday - 0 emails
  • Tuesday - 200 emails, 6 replies
  • Wednesday - 200 emails, 6 replies
  • Thursday - 200 emails, 6 replies
  • Friday - 0 emails, and so on.

An alternative example, with a tool that sends out 30 emails a day, and generates an 80% reply rate:

  • Friday - 30 emails, 24 replies
  • Saturday - 30 emails, 24 replies
  • Sunday - 30 emails, 24 replies
  • Monday - 30 emails, 24 replies
  • Tuesday - 230 emails, 30 replies
  • Wednesday - 230 emails, 30 replies
  • Thursday - 230 emails, 30 replies
  • Friday - 30 emails, 24 replies, and so on.

You’ve got to admit that it looks a lot more consistent, and the engagement generated, seemingly, by your campaigns, is quite impressive. If this keeps up, ESPs should start looking at your emails even more favorably.

Wrapping up

If there’s one thing I can advise, it is to take the IP warmup process seriously. It really has the power to boost your campaigns. This, in turn, will likely translate to improved revenue.

There’s no good answer as to whether manual or automatic email warmup methods are better.

The former is more cost-effective but will require regular effort from you. The latter takes care of everything for you.

As your sending volume grows, automated methods will probably turn out to be more feasible, but, of course, the decision is yours to take.

Irina Maltseva
Irina Maltseva

Previously Head of Marketing at Hunter. I enjoy working on inbound and product marketing strategies. In my spare time, I entertain my cat Persie and collect airline miles.