AfterOffice uses SSL certificates that are issued by Let’s Encrypt for all of our email servers, to support the secure SSL connections. 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 mail client to access your emails with secure IMAP/POP3/SMTP connections on an older device – which do not trust/recognize ISRG Root X1 by default – you will get an SSL certificate expiration warning.
Users who are using the following devices can try to install and trust the new root certificate manually.
If the above does not work for your device, you can opt to:
Reference: DST Root CA X3 Expiration, ISRG Root X1 Certificate Compatibility