Small discussion a month ago (15 points, 10 comments) <a href="https://news.ycombinator.com/item?id=41630461">https://news.ycombinator.com/item?id=41630461</a>
TLDR: they're used as a revocation mechanism for DKIM.<p>non "paywalled" link: <a href="https://threadreaderapp.com/thread/1852021884902138123.html" rel="nofollow">https://threadreaderapp.com/thread/1852021884902138123.html</a>