When we open a previous mail, the dynamic contents are saved even when we don't click on save button ! So we can't have history of sending emails...
The answer of support team = if you open the email (and get to the newsletter editor), it’s expected that you’ll see the Automatic Latest Content block updated. And then as we have an auto-save feature, even if you close it without saving the newsletters, changes will be applied.
So a alert "would you like to save?" is a better way I think