In an email you can't of course since an email could contain one, the other, or a mix. And, in fact, most mailing list emails I read these days do contain code

In an email you can't of course since an email could contain one, the other, or a mix. And, in fact, most mailing list emails I read these days do contain code and code reviews. However, if the user clicked the "reader mode" button, that's a good sign the user thinks this is reflowable text. Firefox's reader mode figures this out. Safari's doesn't.