unable to get a VALID certificate

[lucafolin]

I'm newbie about s/mime certificate, after issuing new s/mime certificate for my email address I have uploaded it into my Synology Mail Plus Client to sign email but it's considered Invalid:

With a tool like Key Store Explorer I tried to check the validity of the certificate in any CRL or CSP way but I could not get my certificate validated!
Is the problem mine or is the CA not working anymore?

[ComputersWithTimo]

They were never valid as the owner of the projects hosts its own CA and it doesn't appear to be cross signed.

"differs from Let's Encrypt, utilizing its own certification authority"
"If CASTLE Platform® CA is trusted"

[joeyoropesa-dev]

They were never valid as the owner of the projects hosts its own CA and it doesn't appear to be cross signed.

"differs from Let's Encrypt, utilizing its own certification authority" "If CASTLE Platform® CA is trusted"

Well if acme doesn't provide free S/MIME globally-trusted certificates, what providers give that option but to be able to get it for free and when you need to renew it, you can get a new certificate without needing to move to paid method. Simply be free - staying free..