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 let a big phishing wave right into someone’s inbox.

This pickiness can be particularly annoying for new cold emailers. 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? Of course, once you’ve found the email addresses of potential leads, you should verify them. But it’s just a first step.

Simultaneously, you need to do a proper IP warmup. It’s a 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 that, both manual and automated. I’ll cover both approaches.

1. Email deliverability in a nutshell
2. IP warmup - the basics
3. Manual IP warmup
4. Automated IP warmup
5. Email warmup never really ends

1. Email deliverability in a nutshell

Email deliverability is the art of getting emails delivered right into the recipient’s inboxes. And not just anywhere in their inbox - you don’t want your emails lingering in someone’s “Promotions” folder, never to be opened anyway. The aim of any effort put into deliverability should be to have every email land in the “Primary” folder of the lead’s inbox. No exceptions.

Only that kind of inbox placement guarantees that most people will see a message. If they see it, they may just open it and even take the desired action (usually - reply). 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’s 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 1000 emails sent without any effort on the deliverability front, 800 were delivered (some to “Primary” 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 revenues. But over time, by implementing the best practices, you should be able to see a visible boost in result.

Main factors impacting email deliverability

The algorithms analyzing our emails are very complex. They look at dozens of factors when assessing our emails. Although we don’t know their entire specification, years of research gave some pretty clear answers on what matters the most:

1) Sender reputation (also referred to as ‘email reputation’ or ‘IP/domain reputation’). However you call it, it’s your reputation in the network. You build it by sending quality emails that don’t bounce or end up in spam folders. Recipients also don’t mark them as spam and they engage with them, replying or clicking on links.

When it becomes a typical pattern, ESPs (Email Service Providers, e.g., Gmail, Outlook) 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) Authentications. To fight spam and phishing, some serious people in the email world came up with authentication methods such as SPF, DKIM, and DMARC. These are sorts of 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 authentications 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. It’s mainly a case when you’re guessing emails or received a list from some shady sources. Mailing lists that people subscribe to to gain access to some resources or free wi-fi in the airport aren’t very reliable.

I can’t emphasize enough the importance of verifying each email before sending, not by sending. Although certainly, sending an email to an 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 of those are reported, the worse for your deliverability.

Verifying emails with Hunter is extremely simple. Log in to your account and head over to Hunter Email Verifier. Enter the email address in question and almost immediately you’ll see its status.

Verifying emails with Hunter

You can also do it in bulk with Bulk Email Verifier. Once the scan is ready, you can download the results and update your mailing list.

Bulk verification with Hunter

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 spammy words and content
  • Avoiding attachments when unnecessary
  • Allowing leads to opt out

2. IP warmup - the basics

Going back to the 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 checkup on you when you’re getting started, give you a green light and move on to the next newbie. Quite 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 when a solid IP warmup can do wonders. It can be done in two ways:

  • Manual IP warmup - it’s when you manually email more and more addresses, maintain the right schedule and gradually increase the volume.
  • Automated IP warmup - there are dedicated platforms that can warm up the IP address for you - for a fee, of course. It requires less effort on your side and also works very well.

Things to do ahead of IP warmup

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

  1. Decide on the sending provider

An important element of sender reputation is the tool you use for sending emails. And I don’t mean the email client itself (e.g. Apple Mail) but the ESP dispatching your emails - Gmail, Outlook, AOL, Yahoo, etc.

When analyzing your emails, spam filters look both at the reputation of your domain, and of ESP of your choice. Even if you’ve build a solid reputation yourself, using a poor provider will drag your score down. It works in both directions - if you don’t have much of a reputation, picking a solid provider to send on your behalf gives you an immediate boost.

Many of you probably use Gmail for sending cold emails. It’s not a great solution but, as the Netflix’s ‘Cherbobyl’ guy would say, it’s not terrible either. The problem is that many spammers also use Gmail for sending their foul messages and it affects the reputation of servers you inevitably co-share.

If Gmail is your choice, consider jumping at least to their premium service called Google Workspace (previously G Suite), priced at $6/month. They’ll allocate you immediately to higher quality servers and will toss in some extra features too.

If you plan to send cold emails in bulk, consider doing so with Hunter Campaigns. We’ve built it to help you automate the cold emailing process, while keeping each email personalized. You can build schedules, customize each email with your own data, and, of course, track how your campaigns perform.

Personalized email campaigns

It takes just a few seconds to connect your Gmail account with Hunter. Hunter Campaigns are free for a single Gmail account.

2. Consider sending from an alternative domain

It’s not a must but could be valuable. If you’re worried about the performance of your emails, it may be worth setting up an alternative domain that you’ll use only for sending outreach emails. It could differ only by the top-level domain - for example if your domain name ends with .com, register one with .io too. Here’s how we did it at Hunter:

Alternative domains for cold emailing

The idea is that cold emails tend to have a higher unsubscribe / spam rate than transactional or even marketing emails. As a result, they could drag down the other messages your company sends - for example, password reset emails or important updates. You don’t want them missing your customer’s inboxes.

3. Authenticate

I mentioned in the previous chapter the important aspect of email deliverability - email authentications. There are three methods in common use but it’s not something you have to choose from. You want to actually have all three set up and, at the very least, SPF and DKIM.

Here’s what each does:

  • SPF tells a spam filter which IP addresses (domains) are authorized to send emails on your behalf
  • DKIM is a digital certificate that confirms the authenticity of your message
  • DMARC combines both of the previous methods and lets you also decide on how to handle emails that fail either test

The good news is that some email providers take care of at least SPF and DKIM for you. Each provider should have the setup process documented as well.

If you’re unsure if you have either method set up, send an email to another email address of yours (if you can access it, even better). Then, on the recipient’s end, look for the details of an original message. In Gmail, for example, click on the three-dots menu in the top-right corner of a message and choose ‘Show original”. You’ll see something as shown below. You’re looking for the word ‘PASS’ right next to the respective method.

Authentication check in Gmail

There’s also plenty of free tools that allow you to check up on your authentication status. Mxtoolbox, with its SPF, DKIM, and DMARC checks, is among the most sophisticated options. also looks up your authentications as you send a test email their way.

4. Craft a non-spammy copy

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 - “Great offer”, “Instant”, “Free”, “Giving away”, “Amazing”, and so on. You get the idea.

The thing is, ESPs hate those words even more than we do, simply because they’re often associated with spam. Try to avoid these and many similar words at all costs. These cold email templates could be a good start.

Hunter Templates

Additional read: How to Write a Cold Email That Converts

5. Show a bit of patience

You know what’s another common pattern of spammers? They register a domain, and an account. 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 emails so you’re not flagged as one of them.

For new domains, it’s recommended to wait about three months so that a news about your new domain spreads out in the network. Then you can slowly ramp up with your warmup emails.

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

First things first, find a group of at least 20-30 people that would be up for sending some emails. These could be your old friends, family members, former colleagues - basically, whoever in your network wields a weapon called an email account. Try to refrain from adding your current colleagues that send from the same domain as yours - it will look fake.

Look in particular for people who use company domains (such as rather than all sorts of gmails and hotmails. The latter are not bad, but the more of the unique domains you can find, the more credible you’ll look in the eyes of ESPs.

Even better if your recipients have emails registered on domains hosted by many different providers - Google, AWS, Yahoo, GoDaddy, Cloudflare, etc. By sending to various hosting providers, you’re teaching each of them that your emails are legitimate and the audience is highly engaged.

If you’re not sure where your peers host their domains, you can look it up in the public WHOIS database. Use, for example, the search on GoDaddy and look for what appears for ‘Registrar”. Here’s an example query for

WHOIS query for

Once you’ve carefully selected your partners in crime, give them a heads up (not with the email you’re about to warm up, though ;-)). Tell them you’ll send them a few emails shortly and you want them to:

  • Open them
  • (if them land in spam) find themand mark them as ‘no spam”
  • Mark each message as ‘Important’
  • Reply (with some genuine answers - these don’t need to qualify for a Pulitzer Prize but should make at least some sense)
  • Forward messages to someone they know

Craft some good looking copies yourself (just text, no images, no links, no spammy words, no attachments). And plan a schedule.

Example IP warmup schedule

Start off slowly. In week one I would recommend sending 15-20 unique emails every day. Try not send them every day 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 a day.
  • It’s okay to skip some days (e.g. weekend). 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 25-30%+ reply rate, you’ll be okay. But the more replies, the faster you’ll have a warmed up IP.

In week two, increase the volume and try to send 30-40 emails every day. It’s okay if someone gets more than one email in a day. Try only not to send them a few emails in a row. Instead, wait for them to reply and reply back.

In week three, aim for about 50 to 80 emails. You’ll inevitably contact the same folks more than once a day but it’s fine.

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

Manual warmup schedule

If possible, try to smooth out the transitions between 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. 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 for 500 emails to be send, 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 well warmed up, a sudden spike in volume may not be viewed favourably.

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 be also used in conjunction - starting off with a manual warmup and then plugging in a tool to finish up a process but there’s 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 “no-spam,” and producing a reply for each. All of this to show to email servers that you send valuable emails and your audience is genuinely engaged. And it works!

Examples of automated IP warmup tools

There’s 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 - e.g., adjust the reply rate or the number of emails sent each day.

Then, they do all sorts of standard actions - sending realistic emails, auto-responding to them, digging them out of spam, or marking each of your messages as an important one. There’s also inbox placement monitoring included in the dashboard and a set of tools for improving deliverability.

2. Mailwarm

Mailwarm homepage

Mailwarm sends your warmup emails to over 1,000 accounts, generating replies and all sorts of positive interactions. If you already have an idea on how many emails you want to send each day Mailwarm lets you configure precise numbers. If you don’t, it will suggest warmup strategies for you. Then, you can sit back and watch the statistics of your campaign in a dashboard.

3. Warmup Inbox

Warmup Inbox homepage

Warmup Inbox exchanges emails between your inbox and the real account of over 4,500 of their members. The platform automatically interacts with your emails in the same way that all competitors do, boosting your deliverability in the process. It also constantly monitors for your presence on blacklists and gives you a headup if you land on one.

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 that I mentioned above, GlockApps, and Sender Score.

I would highly recommend that you try to balance your campaigns, spread them onto several days or even weeks for much bigger volumes. 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?

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, archived. This doesn’t help your reputation much.

You can employ a 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 even more favorably in your direction.

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 into higher revenues.

There’s no good answer as to whether manual or automatic methods are better. The former is more cost-effective but will require from you a regular effort. The latter takes care of everything for you. As the volume grows, the 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.