要先透過 Google Forms 填單等待，等通過後要指定 production server 才會發出來。
要注意的是，Let's Encrypt 發出來的 SSL certificate 會比較短，只有 90 天有效，設計上是依靠自動機制在更新，所以理論上不會有問題：
Certificates from Let's Encrypt are valid for 90 days. We recommend renewing them every 60 days to provide a nice margin of error. As a beta participant, you should be prepared to manually renew your certificates at that time. As we get closer to General Availability, we hope to have automatic renewal tested and working on more platforms, but for now, please play it safe and keep track.
再來是有註冊限制，分成 Registrations/IP address 與 Certificates/Domain 兩個限制：
There are two rate limits in play: Registrations/IP address, and Certificates/Domain.
前者的限制是每天 10 次：
Registrations/IP address limits the number of registrations you can make in a given day; currently 10. This means you should avoid deleting the /etc/letsencrypt/accounts folder, or you may not be able to re-register.
後者的限制是每個 domain 只給 4 個 certificate：
Certificates/Domain you could run into through repeated re-issuance. This limit measures certificates issued for a given combination of Top Level Domain + Domain. This means if you issue certificates for the following domains, at the end you would have what we consider 4 certificates for the domain example.com.