Two weeks ago (11/30/17) our email opens dropped by about 40% according to civireports summary.
We are an extremely small, niche, nonprofit religious site. Our newsletters titles have not changed and are not particular spammy ("Emma Lazarus, Maud Nathan and Alice Menken: Notable American Jewish Women", "Rav Kook and His View on the Modernization...")
Anecdotally, for email accounts that we monitor, we can see that from the past two weeks started going to spam. The accounts we monitor are gmail.
Neither our site nor our IP (shared) are on any blacklist sites.
I've cross posted in stackexchange webmasters from lack of ideas.