On September 30 2021, Let's Encrypt updated their ROOT certificate.
The ROOT certificate is responsible for issuing SSL Certificates by Let's Encrypt.
This change has caused the older ROOT certificate (DST Root CA X3) to no longer be accepted, this has been replaced with ISRG Root X1.
Some Email programs may still be referencing the older Root Certificate being (DST Root CA X3) and causing the mail clients NOT to validate the SSL SMTP and IMAP connections. This may be an issue with older devices that are not compatible with the new ROOT certificate.
For the full press release from Let's Encrypt please see the link below.https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
As a temporary work-around please use these alternate ports for IMAP and POP which can be found below.
Non-SSL IMAP (Incoming) : 143
Non-SSL POP (Incoming) : 110
Non-SSL SMTP (Outgoing) : 587
Please see our email setup guide here - > https://support.panthur.com.au/en/knowledgebase/article/email-setup-guides