Image blocking in email

Ever seen an email appear in your inbox like the one below? We’re sure you have. Where have all the images gone? Email image blocking is a feature or tactic used by email clients whereby images in campaigns are prohibited from showing unless expressly asked to do so.

Missing Images!

What is email blocking and why do email clients block images?

In a nutshell, when your email campaign lands, the email client doesn’t just load images and will only show images in your campaign to readers on your list that enable images to display. Another step to get your message out, annoying right?

Well….yes, but here’s why they do it.

Why email clients block images?


A lot of people still believe that the only reason email clients block images in email campaigns is because they want to save users on bandwidth. This would seem like a compelling reason or argument but if you test how mobile email clients render images, most (if not all) the email clients that we tested allow images to show by default. It would make more sense for email clients to turn images off for mobile email clients if bandwidth was the main motivating factor behind this setting.

Security reasons

The absolute main reason why email clients have image blocking as a default setting is for security reasons. Tech giant Microsoft claims that the reason why their email clients block images is that they want to help their readers to avoid harmful content, avoid potentially offensive content and then also protect them from malicious code. Part of Microsoft’s motivation is also related to bandwidth usage and giving their users the choice on whether they want to download images or not.

Image hosting that isn’t done through proxy servers

A lot of other email clients however do have images turned on by default as a lot of them use proxy servers that host images - instead of getting the images to come directly from external hosts. Doing that allows an email client to check for any malware or viruses before they land in their readers’ inbox. Essentially this mitigates the need for blocking images due to security concerns.

Who are the email (culprits) clients who block images?

When we look at email clients, it’s important to know who blocks images by default and also who allows alt text, or stylised alt text. It’s important because you can look at what email clients the majority of your subscribers use and optimise your email campaigns accordingly.

We’re going to split the email clients for you in categories to make things simpler. (A special shout out to Litmus for the content of the tables here).

Webmail Email Clients

These are email clients that are completely web-based, meaning, users of this email client log into their accounts through their preferred browser.

We’ve added the default image setting by email client in our table below for you too as a handy guide for you.

Office 365 ON YES YES

PLEASE NOTE: that in AOL mail stylised alt text will still show underline alt text no matter what stylisation is applied.

Desktop Email Clients

Desktop email clients are email clients that require a user to download a programme to their desktop to access their email accounts.

Outlook 2007-2019 OFF YES NO
Outlook (mac) OFF YES YES
Apple Mail ON YES YES
Office 365 OFF YES NO
Windows 10 Mail ON NO NO

A few things to note regarding the above:

  • In Outlook 2016 the default image status was turned off.
  • ALT text is shown in Outlook 2007-2019 but only renders after a warning message is shown and also only if images are turned off. It’s important to also note that if images are broken for some reason ALT text will not show.

Mobile Email Clients

Email clients are exactly what they say, email applications that are specific to mobile devices whereby users access their emails via their mobile device.

Yahoo! Mail ON YES YES
iPhone (iOS Mail) ON YES YES
Samsung Mail ON YES YES
Outlook ON YES YES

If you’ve looked at the above information and thought to yourself, “maybe I should stay away from sending any images in my campaigns going forward” - don’t. It’s important to apply the knowledge gained from the above and overlay it with market share information and then further look at the distribution of email clients on your own subscriber lists.

Let’s look at what we know about email client market share in 2021.

Email Client Market Share in 2021

When we looked at the data available in terms of the market share different email clients own, we found some interesting insights.

For more information on the data below, please visit Litmus’s Email Market Share study.

Most notably the Apple iPhone market share increased by 5% (since 2020) and a lot of experts believe it is due to the popularity of their iPhone 12 family.

Email Market Share from Litmus!

Let’s look at open data across email categories

If we look at all opens in the study we can see that opens are spread out as follows, with most people opening their email using Webmail email clients:

  • Desktop: 19.4%
  • Webmail: 36.1%
  • Mobile: 43.0%
  • Other: 1.5%

Let’s split this out a bit more by email client category and look at the top 3 email clients in terms of where people are opening their emails.

Webmail Email Clients Gmail 75% Yahoo! Mail 14.8% 2.4%
Desktop Email Clients Apple Mail 58.9% Outlook 40% Windows Live Mail 0.9%
Mobile Email Clients Apple iPhone 90% Google Android 3.5% Apple iPad 3.2%

This data is great to guide you in the right direction when it comes to email client usage and how you can move forward and optimise your email campaigns. BUT you need to keep in mind that the data is based on average across various industries and there is nothing average about your subscriber list. Use the data above to guide your email design strategies by looking at your own subscriber base.

Can I tell how many of my subscribers block their images?

You are now equipped with enough data to help you make informed decisions to help you optimise your campaigns to account for image blocking as set out by email clients.

So what’s next? How do you know how many email subscribers block their images? The truth is you can’t 100% tell. You can make informed guesses based on the data you have available to you but there are many things that come into play, namely the default email client setting and your individual subscriber’s settings in their inbox

What can I do about this?

It’s really important to not freak out about whether your subscriber is able to see your images or not and rather focus on how you can take measures to account for if they can’t see them.

Enter ALT text...

You may have heard of or seen people talk about ALT text and wondered what it’s all about. Alt text is a way for you to essentially label your images within your campaign to make sure people know what should have shown in certain email blocks when something goes wrong i.e. when the image won’t show, is broken, etc.

If you use ALT text, you are basically telling a system what should show in an email when an image can’t be displayed for whatever reason.

ALT text is your secret weapon to provide context when this happens and also to encourage your readers to adjust their image settings.

Adding ALT text is super easy in our builder, all you need to do is SELECT the image and add your ALT text under the ALT Text bar as illustrated below.

Alt text setting in Mail Blaze!


Whitelisting is another way that you can combat blocked images by expressly asking your subscribers to whitelist your emails. This means you ask them to add you to their safe senders.

Some email marketers include the instructions for how to whitelist their emails in their welcome email so they can encourage their readers to see all the beautiful assets they’ve created from the get go.

Whitelisting shows and proves to your subscriber’s ISP that the email you have sent is safe. This in turn ensures that your subscribers will receive your email campaigns.

Don’t do this

We’ve said it countless times (but here we go again), don’t send image-only email campaigns. We know image only emails can be beautiful, but they can harm your campaign performance and at the end of the day, you’re looking to increase your bottom line. A pretty email at the end of the day is a pretty email, you want a beautiful campaign that is geared towards helping you reach your business goals. Don’t choose design over practically - ever.

We hope you understand more about email image blocking and how you can account for email clients that don’t support images by default. Let us know if you have any more questions in this regard in the comments below.


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