LetsEncrypt SSL Certificates that are currently in use will still show the old, now-invalid certificate in their CA Bundle. To get emails and web traffic working as normal again, the most straightforward solution is to uninstall and reinstall your current certificate, but download the new CA Bundle.
Update your email program to the following 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
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.
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