AfterOffice uses SSL certificates that are issued by Let’s Encrypt for all of our customers' websites and secure webmail login pages. Let's Encrypt has two main root certificate:
As of Sep 30, 2021, Let's Encrypt has retired and discontinued the use of DST Root CA X3. If you are using an older device – which do not trust/recognize ISRG Root X1 by default – to access your HTTP websites or webmail login page, you will get an SSL certificate expiration warning. Read more: Let's Encrypt DST Root CA X3 Expiration.
To resolve this issue, users are advised to update their device's software/system in order to be compatible with the new root certificate, or, use a newer device with an updated OS/system.
If system upgrade is not available, users who are using the following devices can try to install and trust the new root certificate manually.
Reference: DST Root CA X3 Expiration, ISRG Root X1 Certificate Compatibility