Craig Mod writes "Oh God, It's Raining Newsletters" (2019), which is a delightful piece of writing with many links that you should read in full, but I will quote a few snippets:

"I fear we’re entering an era of newsletter fatigue…" "the state of newsletters and email, in 2019: Things are unexpectedly amazing" "there’s something about the framing of email — the inbox, that weird neither-here-nor-there networked space" "These newsletters are the most backed up pieces of writing in history, copies in millions of inboxes, on millions of hard drives and servers, far more than any blog post." "They’re everywhere but privately so, hidden, piggybacking on the most accessible, oldest networked publishing platform in the world."

via nicoth