Direct-delivery: incoming mail LetsEncrypt failure (prop.ltcmp.net.key is wrong) & nondelivery due to this?

Interesting, why do you think this is obviously seen as just a placeholder? Should the acronyms be self-evident? If so, then perhaps let’s make it even more self evident with something more like: <REPLACE_THIS_WITH_LETSENCRYPT_DOT.KEY_FILE_PATH.key>