Some email addresses are no longer being delivered and are not in the spam folder.

My observation is that the email provider for those recipients is rejecting your email as unsafe. Some email providers, such as Gmail, are very security conscious and prefer to remove possibly unsafe emails immediately with no response. Knowing why it is being done is a project in itself. This happens more frequently with self-hosted email accounts as their settings are often not as secure as those obtained from an email provider.

I misspoke, or perhaps the translation changed the meaning. It's me who isn't receiving emails from certain people (that I used to receive). I discovered that it's only emails from orange.fr that aren't arriving.