Is it done with the "loading" attribute[1] for the img tag? (i.e. lazy loading)
(in which case I assume it's only useful in some instances, since viewports might be of various sizes and there aren't that many emails that are long enough[2] to involve much scrolling for example.
Presumably the server just delays the response for x seconds, with the assumption that any in-flight network requests are cancelled by the email client when the user closes the window or app.
In general, email clients are really, really, really dumb. Everything gets loaded at once. So unless it was an HTML attribute that was available in the 90s, it's better to assume the magic is happening server side.
(in which case I assume it's only useful in some instances, since viewports might be of various sizes and there aren't that many emails that are long enough[2] to involve much scrolling for example.
[1] https://developer.mozilla.org/en-US/docs/Web/Performance/Laz...
[2] https://sleeknote.com/blog/ideal-email-length