How does image blocking work in email clients?

You spend an hour on a beautifully designed email. A big hero image, product photos, a colorful button made from an image. Then a B2B subscriber opens it in Outlook on a corporate laptop and sees a column of empty boxes with tiny broken-image icons. That's image blocking: a deliberate security feature where email clients don't load external images until the reader explicitly allows them.

But Here's how it works. When you send an email, the images aren't embedded in the message itself. They're hosted on a server, and the email contains URLs that point to them. When the client loads the email, it has to make separate requests to fetch each image from its server. Many clients, particularly desktop Outlook and corporate webmail, block those requests by default. The client shows the email's text and layout but leaves image slots empty until the reader clicks a prompt like "Download pictures." The primary reason is privacy: loading an image lets the sender know the email was opened and reveals the recipient's IP address. It's also a defense against malicious image payloads.

The practical implication is that your email has to communicate clearly without images. Every image needs a descriptive alt attribute so blocked subscribers see text in place of the image. A hero image that just says alt="" shows nothing when blocked. One that says alt="New arrivals: 30% off this weekend only" still delivers the message. The alt text guide covers formatting rules for different email clients, since some render styled alt text and others don't.

Image blocking hits B2B senders especially hard. Corporate IT policies often set image blocking as the default for all employees, and there's no way around it on your end. That means your email's CTA, key offer, and primary message should never live inside an image alone. Use HTML text for your headline and offer, and use images to reinforce, not carry, the message. If your "Shop Now" button is an image with no alt text, blocked subscribers have no idea what to click.

The easiest way to know how your email actually looks when images are blocked is to test it before you send. Turn off image loading in your own email client or use a preview tool that shows the blocked-image view. If the email still communicates your message clearly without any images visible, it's ready to send.