It’s a result of DST Root CA X3 Expiration (September 2021) - Let's Encrypt. It caught a few people off guard and the issues that it caused for Discourse self-hosters are subtle enough that it got overlooked. And there are still issues that need to be resolved.
I suspect that they stopped being handled October 1.
I never figured out how to get SES working. An emergency stopgap would be to set up outgoing mail with mailgun.