Hot for Security is Bitdefender's security news blog covering breaches, cryptocurrency theft, malware, and vulnerabilities.
HN links it for news reports on breaches and security flaws, sometimes as a non-paywalled alternative source.
https://hotforsecurity.bitdefender.com/blog/mathematician-impersonates-google-founder-to-point-out-dkim-flaw-4061.html
Once that key is known, it can be impersonated. Regular rotation is a practical mitigation strategy and I like that Sendgrid took it on ahead of the game.
Since they are sending, they can create a new key on the second domain, tell new emails to use it without impacting anything in transit by leaving the old one active until it is changed for rotation.
There's a non-paywall article on the subject at:
https://hotforsecurity.bitdefender.com/blog/south-africas-postbank-is-replacing-12-million-bank-cards-after-major-security-breach-23503.html