What is an ESP (Email Service Provider) and why do you need one?

What is an ESP (Email Service Provider) and why do you need one?

An Email Service Provider (ESP) is not what you might think at first: it’s not a service like Gmail, Yahoo, Hotmail, Outlook or others, although they too provide what you might call email ‘services’. It also has nothing to do with your Internet Service Provider (ISP).

TL;DR Simply put, an ESP is a company that provides a range of services for sending bulk email to your customers or ‘subscribers’, specifically for marketing purposes.

There are literally hundreds of email service providers out there, each one boasting more features than the next. Examples include ConvertKit, MailChimp, ConstantContact, Aweber - the list goes on! It can be very hard to decide which one to use because of so many features and distinguishing factors.

How do you know which one is best for your specific needs without trying them? Who has time to try all of them?

Typically, any decent ESP will offer at least the following services:

  • Allow you to maintain a list of subscribers (allow your web visitors to subscribe to your list on your website or even import them yourself)
  • Create email campaigns (or broadcasts, e-blasts, newsletters or whatever you may want to call them)
  • Send these campaigns to your subscribers in bulk.
  • Customise the email templates, to include your logo and colour theme for example.
  • Provide reporting facilities to measure the impact of your emails, such as open rates, click rates, unsubscribes and more.

More advanced offerings you may find include:

  • List segmentation - allowing you to send emails only to a subset of your subscribers based on criteria you define (location, preferences, tags, custom fields)
  • A/B testing - for instance allowing you to dynamically swap the subject line and measure which of the two subject lines are better in order to optimise open rates.
  • Automation - reacting to various events (such as visitor clicks or actions) and performing certain tasks as a result (for example: when a subscriber clicks on a product link, tag them with an interest in that product, etc.)

In essence, an ESP does more or less the same thing as the next one… save for a few things that they may have extra or do differently, that they can use in their marketing to target specific users. 

The aspects that really matter can be:

The price

Many ESPs have various pricing tiers, largely defined by the number of subscribers you want to send to. Some of them even have a free tier for up to say, 1000 or 2000 subscribers with a number of free emails every month. 

Others don’t offer a free tier and their starting price could even be considered high. It can be quite difficult to be convinced to sign up for one, particularly when you are just starting. Don’t let that be the main consideration though!

The features

As I said, since most of the ESPs offer more or less the same features, taken broadly, there are however some that you might lean towards, depending on your needs. You may want to be able to create beautiful-looking emails, so visual customisation might be an important factor for you. If you are more artistic in nature or you don’t know how to code, you may appreciate it if the service provides some pre-designed templates you can personalise. Automation may not be something you’re interested in.

However, if you’re more of a geek and want to hand-code everything yourself, you may not care for predefined, funny-looking templates that other people have used, and may want to go for something simple but efficient. On the other hand, you may care very much about automations, tagging, content upgrades and why not, integrations such as APIs to be able to connect and talk to other systems (such as your website or CRM).

Ultimately, one size doesn’t fit all.

The limitations

Most ESPs have some limitation or other, be it missing features they did not implement or things they just ‘don’t allow’ due to various reasons.

They won’t tell you these upfront, you need to dig deep and find these so make sure you read the terms and conditions before you commit. Nobody wants to advertise their list on their site and build a subscriber list for months, only to discover the ESP doesn’t support their particular market or type of emails they intend to send.

For instance, many bloggers try to create websites to advertise affiliate products as a means to monetise their blog. But for example, MailChimp does not allow you to send emails related to affiliate products - it’s right there in their policies. 

So if you start building a list but don’t send an email to that list right away, you may never realise until you actually try to send something later - when it will be too late. You may discover your account is blocked when you try to send a newsletter or campaign that includes affiliate links.

Most email service providers also typically block your sending until they’ve assessed your account, checked your list (if you imported one from somewhere else, such as your email program), or, if you have no list, they may tend to ask you a few questions and require that you show them your website. This is a way for them to evaluate if you’re likely to be a liability for them - in case you may be a spammer or have some suspicious activity (such as pornography, gambling, illegal drugs and so on).

Some ESPs - since they have so many accounts with millions and millions of emails in them - can analyse a list you import against other lists in their other accounts and if they notice that a very high percentage of addresses on your list matches addresses in another list or account, they will immediately flag it as a list you’ve potentially purchased from somewhere.

If you’re migrating from another ESP, ConvertKit asks for some statistics from your old service, to see if there are any chances you might be importing a bad list with bad unsubscribe rates or too few open rates.

Why do you even need an ESP?

If you’ve ever tried to send email to more than 30, 40 or 50 people at once you may have hit the limits of your ISP or email provider (Gmail, etc.) and were denied sending, they bounced back or worse - they got you blacklisted.

In order to prevent spam and abuse, regular email services don’t allow you to send email to too many recipients at a time. Some have a daily limit or a per-message limit.

Gmail has a limit of 150 messages per day when using a remote mail client or 500 emails per day through their SMTP. This applies whether you include everyone in the ‘To:’ field (which you should NEVER do), in the ‘CC:’ field or in the ‘BCC:’ field.

Sending an email to everyone at once may sound like a solution to organise a quick company picnic, and you may even get away with it for a while, but when you’re talking marketing messages and potentially hundreds or thousands of recipients, it’s impossible to do it like that.

That’s why it’s necessary to have some kind of system that will send the same message to all subscribers, as fast as possible, as reliable as possible, without showing to everyone who else received it and also pleasing the old’ Spam filter. Pretty hard to do!

Sure, you may try downloading some free software that runs on your computer or your server and sends the messages in batches (phpList for example). 

I did that a good few years ago for an e-commerce client. 

My client had a list of about 11.000 subscribers and we had been sending those subscribers newsletters almost every week with offers of their products, promotions and so on. The results were quite poor - we didn’t know how many people were actually opening our emails, not to mention not getting many orders! From 11.000 subscribers, that was appalling.

After a while, we decided to get serious and we signed up for an ESP, we imported all of our lists and sent out a newsletter. We could not believe the response! Yes, there were a few bounces, a few spam complaints and best of all, a huge number of orders in a single day! We then realised, our subscribers were not even receiving our emails. It was like we had been shouting alone, in the void.

Even if you do manage to circumvent your ISP or email provider, what do you think Gmail or Outlook do when they notice a sudden flood of emails arriving in a short interval from a single address, from an unknown IP address? They will think some computer was hacked and a robot is sending spam and will immediately start rejecting those messages or sending them straight to the junk folder.

I am very much a DIY kind of guy but email delivery is one thing that you just cannot do by yourself - even if you pull it off technically, in the end, it won’t be reliable and it might get your address blocked - not because the software is not able to send all those emails, but because of the previously mentioned limits.

ESPs on the other hand, may have pre-established relationships with email providers. They are known to be ESPs - that’s what they do, and may have agreements with Gmail and the like, to ACCEPT their messages or Gmail are aware that they are an ESP and have ways to hold them accountable for the emails originating on their network. 

They also do what is called ‘IP address warmup’ and ‘IP reputation management’ where they maintain hundreds and hundreds of IP addresses on servers that send email on their behalf. Those servers and IP addresses have a good reputation for sending legitimate email, historically, and are trusted by email providers. Each domain name you send an email with is pre-screened and authorised (with DKIM, SPF, etc. - we will discuss these in another part of this series).

But your IP address (which could be randomly assigned to you by your ISP every time you connect to the internet), may have a bad reputation, or be on some blacklist somewhere due to bad activity by the previous user who had it. Simply sending email from that blacklisted IP could get all of your emails banned and straight down the drain and could get your email address or worse - your domain name blacklisted!

As mentioned in the next section, you will see that there’s a reputation to be maintained with email service providers and an accountability protocol.

Deliverability and reputation

This is I think one of the most sensitive and distinguishing factors and not immediately obvious. Deliverability is the ‘rate of success’ at which an ESP can deliver your emails to your subscribers’ inbox. You will be surprised to find that this is not 100% almost for nobody. There’s no way to really prove the deliverability of any particular ESP because it doesn’t depend entirely on them. 

If you try to send an e-mail to a few hundred subscribers, all on Gmail for example, it depends on Gmail to actually deliver those messages to their users’ inboxes - or, if their spam filter doesn’t like them, deliver them straight to the Spam folder. Likewise, your ISP may deny the messages if they go over certain thresholds or contain suspicious text, links, attachments and so on. Spam filters work with a multitude of signals to assign a spam score and determine what kind of message an email is. 

Further, if historically, your emails don’t have a high open rate, email providers might start to deliver more of them to Spam in the future, even though you have not started doing anything bad or sending illicit emails - these are just various signals being taken into consideration and affecting deliverability.

So as I was mentioning on the previous point, ESPs try to pre-screen your account before they actually let you send email through their network because they try to protect their good reputation with email services (such as Google, Hotmail, Outlook and others).

If they let a bad account send spam to thousands of subscribers and they get blocked, it will impact all of their accounts and ultimately their business, so they have to be very careful. 

That’s why, most email services providers stay in constant contact with email services (such as Gmail, etc.) to make sure they don’t upset them. Whenever someone marks a message as Spam/Junk, Google not only moves that message to Spam - they take note where it came from, through which ESP and they report back to them, flagging them up and asking them to clean up their act - in this case, notify their sender’s account. In serious or repeat violations, even block the sender’s account with no chance of recovery.


As you can see, there’s much more beneath the surface of email service providers - it’s not just sending a bunch of emails to a bunch of subscribers. There are features to offer, improve, market, email providers to appease, reputations to uphold and money to be made. Not all sizes fit all and the devil may be in the details when it comes to compliance, acceptance, deliverability, content you’re allowed to send and more. 

It pays to sign up with a reputable service that takes good care of all these aspects.

I hope this intro to ESPs helps shed more light on things to look out for when choosing the one you’re going to give your subscribers to, for potentially years!


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