Usually this type of error suggests that your mail server is not presenting the Intermediate CA certificates. These are certificates which prove that the entity which signed your certificate (Let’s Encrypt Authority X3) was in turn trusted by another entity and so on back until it reaches a “root trusted” CA certificate. Some clients can guess or cache the right ones, but not all, so you should ensure your mail server software knows to present them.
If you think your mail server is already presenting the Intermediates, try opening that Details tab in the Certificate Viewer window you showed, and screenshot that so we can see it.
Depending on how you got certificates issued (e.g. using certbot?) the extra data you need may be in a file named chain.pem next to the cert.pem you already copied across. But we can’t tell you how to configure your mail server without a lot more details, try reading the documentation for mention of how to configure “Intermediate CA certificates” or a “certificate chain”. If it says you can just run all the certificates, including yours together in a single file, certbot already provides that as fullchain.pem