HOW WE REDUCED THE IMPACT OF ZOMBIE CLIENTS

Summary from the TLDR Web Dev newsletter

"Zombie clients" are abandoned servers that continuously and unsuccessfully request certificate renewals from LetsEncrypt, wasting resources. LetsEncrypt implemented a "pausing" approach, temporarily limiting an account-hostname pair's ability to request certificates if repeated validation failures occur. This system included a self-service unpause feature, allowing users to quickly resume certificate issuance if they resolve the underlying issues.