Jetpack subscriptions stopped

After years of seemingly perfect operation, yesterday I noticed a post on my site did not seem to send out the normal Jetpack based emails to my site subscribers. At least not to the two different test users I have signed up there anyway.

I’ve checked all the basics such as Jetpack being connected to WordPress, everything is up to date etc. I’ve poked around the various Jetpack settings on my site and at WordPress but can’t really see why this should have suddenly stopped working.

Any ideas where I can look next?

https://www.ads-software.com/plugins/jetpack/