This usually means there is a certificate being encountered that lacks an authorizing root certificate, or is missing some other component that is usually present for validating the certificate itself. Do you see this only when delivering to specific hosts?

Hi for the moment I saw this on specific host, yes, because emails are not delivred to them

But I'll check the logs If I see this else where.

Thanks for your follow up on this. You can likely work around this by enabling the certificate check bypass in Settings>Protocols>SMTP Out but ideally the remote server should correct their certificate problems. Let me know if you have any questions :-)