Problem with certificate

Hi,

I can't figure out why, I have this new installation on my customer, but I cannot undestand why the "cacert.der" resulting (made as always done on other installations) has this in the "Certification Path" tab:

"The issuer of this certificate could not be found"

For the other certificates, if I open it I get "The certificate is valid" (or similar, I'm translating from italian)

Can you help me on this? I've spent DAYS, trying recreating certificate in Zimbra in many ways. Outlook give error and asks for confirmation using that certificate.

Zimbra version is: Release 8.8.6.GA.1906.UBUNTU16.64 UBUNTU16_64 FOSS edition.

Zextras 2.6.3

Thanks a lot for any suggestion on this.