I am not really sure about what certificate you refer to - for sending emails, I am not aware of any certificate.
On https://community.openhab.org/ we use let’s encrypt and the certificate looks alright to me:
The message occcurs when i open a summary email, it is casued by the expired certificate, and from my own trying to set up a linux email server is to do with the DKIM record on dns hope that helps you to fix it.
Ok, looks like we had an IPv6 issue and lets encrypt could not route to our forums to verify our renewal. I have temporarily removed IPV6 addresses from our DNS, and was able to renew our cert: