Image blocking
Mail programs not loading an email's remote images until the reader allows it, either by default, by the reader's setting, or because the sender looks suspicious.
What it means
Remote images are fetched from the sender's servers, which reveals that the message was opened, so mail programs restrict them. Classic Outlook blocks automatic picture downloads from the internet by default. Gmail shows images automatically unless the reader changes that setting, but asks before showing them when it considers a sender or message suspicious, and it scans images itself before display so senders cannot learn the reader's location or set cookies through them.
Emails should therefore read well with images off: real text rather than text inside pictures, alt text on every image, and a layout that holds together without them. Image blocking is also part of why open counts undercount readers.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Microsoft Support: Block or unblock automatic picture downloads in classic Outlooksupport.microsoft.com
- Google, Gmail Help: Turn images on or off in Gmailsupport.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes