How image blocking works
Images in an email are not inside the message. The message contains addresses pointing at files on a server, and the mail client fetches them when it displays the email. Because that fetch tells the server the message has been opened, and reveals something about the reader, many clients refuse to make it automatically. They show placeholders instead, with a bar offering to download the pictures.
Behaviour varies. Some clients block remote images until the reader clicks, sometimes permanently and sometimes only for senders not in the address book. Others load everything, but through their own servers rather than yours. Apple’s Mail Privacy Protection does the second: it fetches images whether or not the person actually read the message, which is why opens recorded from Apple Mail cannot be taken at face value.
Why image blocking matters
The obvious cost is a broken-looking email. A template where the headline, the offer and the button are all pictures collapses into a column of empty rectangles, and the reader has no reason to grant permission to see what turns out to be an advert.
The measurement cost is less obvious. Open tracking works by loading a single tiny image, so a reader who blocks images and reads the whole email is never counted as an open, while a client that pre-fetches images counts opens that never happened. Both errors sit inside the same open rate, pulling in opposite directions, which is why clicks and replies are the sturdier numbers.
Common mistakes with image blocking
Building the whole email as one large image is the worst version, and it is still common because it is quick — a designer exports the artwork and drops it in. With images off there is no message at all, filters have no text to read, and screen readers have nothing to announce.
Leaving alt text empty is the next one. Alt text is what a client shows in place of a blocked image, so a missing description turns into a blank box while a written one still carries the point. Relying on a background image for contrast is a third: when it does not load, dark text can end up on a dark panel, or white text on white.
How to act on it
Write the email so it works with the pictures switched off. Headlines, prices, offers and calls to action belong in live text, with images adding to the message rather than carrying it. A simple test is to disable images in your own mail client and read the result — if the email no longer makes sense, neither will it for a good share of your list.
Give every image a meaningful description, using the same discipline as alt text on a website, and set a background colour behind any image so the layout does not fall apart when it is absent. Buttons should be built from text and a coloured cell rather than exported as pictures. On slow or metered mobile connections this is a speed decision as much as a rendering one: a light email arrives readable while a heavy one is still loading.