AND THERE ARE PROBLEMS!!!!
read about it HERE!
On May 4th, 2021, Let’s Encrypt began issuing certificates with a trust chain that includes its ISRG Root X1 certificate cross-signed by IdenTrust’s DST Root CA X3 certificate. On September 30th, 2021, the DST Root CA X3 certificate will expire. Most devices trust the ISRG Root X1 certificate directly and will be unaffected by this expiration.
As a result of this, some clients and users that are connecting from older devices are experiencing connection issues. The devices that are experiencing such connection issues are older devices and so far are identified as follows:
Blackberry less than 10.3.3
macOS prior to 2016
iOS less than version 10
Windows XP (with Service Pack 3)
PlayStation 3 or 4 with firmware less than 5.00
Android 7.1.1 and earlier (of which some certificates have already expired)
Kindle less than 3.4.1
Amazon FireOS with Skill Browser
If you are on a newer device not in this contained list, this will not be affecting you.
If you are experiencing issues connecting to your emails from an unsupported email client, change to webmail temporarily. IE: yourdomain/webmail
Further information can be found here: https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/