Email image blocking is when a mail client declines to download the images in a message until the recipient asks it to. It happens for privacy reasons — every remote image is a request to a server that reveals when and where the message was opened — and for bandwidth reasons on mobile.
The practical consequence is that a meaningful share of your recipients see a version of your email with rectangles where the pictures should be, and some of them never see anything else.
Who blocks and who does not
Gmail caches images through its own proxy and displays them by default. It is not blocking, but the proxy has its own effects: it strips some CSS, ignores certain attributes, and means your image is served from Google rather than your server.
Outlook desktop blocks external images by default in many configurations, and enterprise policy frequently enforces it regardless of user preference. This is the most common source of broken-looking email in B2B.
Apple Mail loads images by default, but under Mail Privacy Protection it loads them through Apple's proxy at a time of its choosing rather than when the message is read.
Corporate security gateways frequently strip or rewrite images before the message reaches the user, independent of the client.
Add it up and the honest planning assumption is that a substantial minority of opens happen with images off, weighted heavily toward business audiences.
Design so the blocked version still works
Never put essential information only in an image. The offer, the price, the deadline and the call to action all belong in HTML text. An email whose entire message is one graphic becomes a blank rectangle, and it is also the classic spam pattern — filters cannot read images either, so a message with no readable text looks like an attempt to hide something.
Write alt text that replaces the image. Not "hero-banner-final-v3.jpg", and not "promotional image". If the graphic says "30% off boots until Sunday", the alt text says that. Alt text is the version many people read, so write it as copy rather than as a label.
Style your alt text. Set colour, font size and weight on the <img> tag itself, and most clients will apply them to the alt text when the image does not load. A styled alt line reads as intentional; the browser default reads as breakage.
Set width and height on every image. Without them, blocked images collapse to a tiny placeholder and your layout folds. With them, the space is reserved and the alt text sits where the image would have been.
Give image cells a background colour. If your logo is white artwork on a transparent background, the blocked state is white text on white. Set the containing cell to your brand colour and the alt text stays visible either way.
Use bulletproof buttons. A button built as a table cell with a background colour and text inside always renders, whatever happens to images. A button that is a JPEG disappears entirely, and it is a call to action, which is the last thing you want to lose. This is the single highest-impact item on the list.
Never use an image as the background for text. The text lands on whatever is behind it, which is usually white.
The preview-pane test
Here is the quick version of testing this. Send yourself the campaign, open it in Outlook with images disabled, and ask one question: from what is visible, do I know what this email is offering and what to do next?
If yes, you are finished. If no, the fix is moving one piece of information out of an image and into text.
Add it to your pre-send checklist alongside dark mode, since both are about how clients transform your design without asking.
What this does to your metrics
Image blocking is why open rate is unreliable. The open pixel is an image; blocked images mean uncounted opens. Combined with Apple's proxy inflating opens at the other end, the number is distorted in both directions by amounts that vary with your audience mix.
Click rate is unaffected by blocking and is the metric to trust. See email engagement metrics for what to watch instead.
Frequently asked questions
What percentage of people block email images?
It varies enormously by audience. Consumer lists on Gmail and Apple Mail see most images load; business lists on Outlook and behind corporate gateways see far fewer. Plan for a substantial minority, and higher in B2B.
Does alt text help deliverability?
Indirectly. Filters read text, not images, so an image-heavy email with little readable content scores worse. Alt text adds readable content and means the message still communicates when images are off.
Should I avoid images in email entirely?
No. Images sell products and carry brand. The rule is that the email should still make sense without them — essential information in text, buttons built from HTML, and alt text that carries the message.
Why does my email look broken in Outlook?
Usually blocked images with no width, height or alt text, so the layout collapses. Outlook's rendering engine has other quirks too — see why your email looks broken in Outlook.
