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:

• DST Root CA X3: An older root certificate that are trusted and recognised by older devices (e.g. Windows XP prior to SP3, MacOS below 10.12.1) and older mail clients (e.g. Windows Live Mail)
• ISRG Root X1: A newer root certificate that are trusted and recognised by newer devices (e.g. Windows XP SP3 and above, MacOS 10.12.1 and above) and newer mail clients

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.

• Windows 7
• MacOS version 10.12 or older
• iPhone iOS

If the above does not work for your device, you can opt to:

• Update your device's software/system in order to be compatible with the new root certificate, or
• Use a newer device with an updated OS/system, or
• Disable SSL connection on your mail client (not recommended).

Reference: DST Root CA X3 Expiration, ISRG Root X1 Certificate Compatibility