Elevated Rate of 503 Service Busy Responses

Starting 0900 UTC on 9 May, Let's Encrypt's API has returned an elevated rate of 503 Service Busy responses, which we return as a protection measure when request volume is high [1].

While absolute numbers are still low (<1% of requests) , we expect to return zero 503 Service Busy responses most of the time.

Clients receiving a 503 Service Busy are given a Retry-After header which indicates how long they should wait before retrying. Many clients do not respect this header; please check that yours does.

We have been investigating the source of this issue, and will continue to do so until we get it resolved.

[1] https://community.letsencrypt.org/t/new-service-busy-responses-beginning-during-high-load/184174