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  • Writer's pictureBen Erdelyi

The Difference Between Email Providers And How To Avoid Their Spam Folders

From our “How to pick up an email marketer” column

Old School vs Modern Email Providers: How To Avoid The Spam Folder?

Email providers…

Even the phrase sounds so boring, my first reaction is to reach for the ice cream and start crying.

But as it happens with everything in life: once you start understanding it and get better and better at it, it actually becomes fun.

Let’s break it down step by step.

There are 2 major groups of providers:

old-school (AOL, Yahoo, most others) and modern (Gmail and Hotmail).

Before jumping into the details of each email providers’ spam folders and the juicy stuff, let’s get to know some key information about the main players.


  • Launched: 2004

  • Parent company: Google Inc.

  • Amount of users: 1.4 billion (April 2018)

  • Max. email attachment limit: 25 MB

  • Mailbox storage: 15GB

  • Supported Languages: 71



  • Launched July 4, 1996 (as Hotmail) July 31, 2012 (as

  • Parent company: Microsoft

  • Amount of users: 1.4 billion (April 2018)

  • Max. email attachment limit: 25 MB

  • Mailbox storage: 15GB

  • Supported Languages: 106

Extra Notes: As per usual by Microsoft, they made a mess of creating a clear name for their product.

Just to make it clear: There used to be a thing called as an email provider, which is now So, if you want a new email address from Microsoft, it will end with

Outlook has recently made some big changes to their spam filters in order to prioritise personal messages for their customers, who are mostly business people.

That means that you will always see a lower inbox rate for Outlook than for any other provider, naturally. As a result, your open and click rates will suffer too.

But you can’t just walk away from your Outlook subscribers, as most of the time this will be the second most important email provider after gmail, in terms of number of users within your subscribers.


Yahoo! Mail

  • Launched: October 8, 1997

  • Parent company: Verizon Communications

  • Amount of users: 225 million active monthly users (February 2017)

  • Maximum email attachment limit: 25MB

  • Mailbox storage: 1TB (the biggest out of the main players)

  • Supported Languages: 27


AOL Mail.

  • Launched: March 1993

  • Amount of users: 24 million  (2012 data)

  • Maximum email attachment limit: 25MB

  • Mailbox storage: Unknown

  • Supported Languages: 27

  • Parent company: Verizon Communications

Extra notes: As you can see, Yahoo! and AOL now have the same company, gaining a bit of market share.

Verizon acquired AOL in 2015 and then Yahoo! as well in 2017, so it is still a relatively new project.

AOL and Yahoo! now make up a new division within the parent company called Oath Inc. or Verizon Media.

Fun Fact: In 1999 during the dotcom bubble, AOL was valued at $222 billion but 16 years later Verizon acquired it for only $4.4 billion. Judge?

Now that we got the dry facts and figures out of the way, let’s get to the main bit.

How does each provider decide what goes to spam?

The above picture shows a breakdown of how different email providers determine whether or not you get in the inbox.

The old-school…

When old-school email providers got started, such as Yahoo, they mostly intended their spam filters for businesses in the 90s.

Big office with a massive machine in the basement of the building that provided the internet with a specific IP address.

Nowadays, it is much more complicated.


When you write an article on your laptop in a café, you will have a different IP address than the one at home, making it more difficult for old-school providers to determine who you are, due to the different location.

And this is where initially Hotmail was revolutionary (and then this was followed and improved by Gmail):

Hotmail put much more emphasis on your subscriber’s engagement (opens, clicks, marked as spam), then your reputation.

The main reason for this is that nowadays it is relatively easy to change your IP address and pretend that you are in San Diego, California, while you are actually in Prague, Czech Republic.

That means that if your initial IP address was blacklisted from everywhere, now you can just change this IP address and build up your reputation again.

I think one of the most important aspects of the graph above is that the actual content is equally important for all email providers — like 5/10 importance.

Some of us may have thought that you need to create beautiful emails with stars coming out and lightsabers swinging around to make you stand out as much as possible.

The truth is, a lot of times your emails that have more than one picture and some design in there will often bounce, or get cut off because you will be way over the threshold of 102 KB email size.

In the new age, your domain reputation and your subscribers’ engagement means everything.

Domain reputation will be determined by how many people mark your emails as spam, how many report you, and how often do your emails bounce.


An example…

There was one time when we got a new client who did not do much email marketing before, and at first, we set up the most important flows in Klaviyo.

Browse abandonment, abandoned cart, new customer thank you, returning customer flows, win-back, re-engagement, etc…

That increased the number of emails sent out by our domain from zero to 2,000 per week, which was already a big increase but it is still okay.

However, we also started sending out warm up email campaigns to all their active subscribers without closely monitoring how many that consists of.

At first, there were only about 1,000 active subscribers there, but for a number of reasons, it skyrocketed to about 10,000 in a matter of weeks.

This meant that the average amount of emails sent jumped from 1,000 to 7,000 in about 5 days, leading Klaviyo to block our account.

We also have clients with lists of 100,000–1 million subscribers so we did not think it was a big deal to use the entire list while it’s “small”.



Rather than meaningless numbers, you gotta think about percentages.

With this example, all Klaviyo noticed is a 600% increase in email volumes — no wonder it led to the spam folder.

Work your list gradually.


Back to your domain reputation: how do you strengthen it and how do you get into the inbox with different providers?

Old-school email providers rely on your IP reputation, a.k.a. third-party sources.

Klaviyo comes in here massively because if you keep sending a shit load of emails and you keep getting low open rates, Klaviyo automatically downgrades your reputation which will see you get a much worse inbox rate on Glockapps.

So basically, if you end up in the spam folder for these old-school providers, you were most likely blacklisted and/or been given a really bad IP reputation.


Switch IP address.

That’s it.

(However, I must note here that if you wanna do well in the long run, you might want to start sending some actually good quality emails.)

What about modern email providers?

In comparison, the two modern providers (Gmail and Hotmail) decide based on your sending domain reputation and receiver engagement.

Recently we have seen Hotmail turning up the fucks given in their spam filters, leading to much more people ending up in this place of doom.

Changing sending domains is much more difficult than changing IP addresses, so for Gmail and Hotmail users, it is essential to send relevant, targeted, and high-quality emails.

That means you need to clean your lists properly, make sure to segment your email campaigns like crazy and pay attention to your KPIs.


Email deliverability is one of the biggest challenges to email marketers.

While trying to bring an amazing ROI (so you can keep your clients) we also have to take account of the long run by managing a sustainable inbox rate.

Some email marketers go all-in, gung-ho in the first month, sending 1 or more email every day and get good results.

The problem with that is that you completely tire out your list and even make a big percentage of them angry at you, and on top of that, you will even get a shitty sending domain reputation from providers.

Doing email marketing in ecommerce is, therefore, a little bit like being the manager of FC Barcelona.

You have to bring instant results while also thinking about the impact of your actions on the long run.

Thanks for reading!

Ben Erdelyi

Account Manager | Budai Media


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