Re-added certificate leads to unknown authority

Messing around with **Keychain Access** on **macOS Sierra**, it seems like I have accidentally deleted **trusted root certificates** in _Keychain Access_. What I did was opening a ***.p7s** file to add a **S/MIME** certificate. This threw an error: An error occurred. Unable to import the certificate. Error: -26276 , but still the certificate ended up being listed in _Keychain Access_. Next, opening **Mail** and creating a new Email I was able to use the certificate. I decided to delete the certificate from the Keychain and re-add it in order to resolve the error. Therefore I deleted all entries containing the name of the trusted source I have used for my certificate (I believe). Now adding the same certificate again, still an error appears but the then listed entry says: This certificate was signed by an unknown authority, and Mail doesn't let me use it anymore... I have also created and added a new certificate with the same authority leading to the same result. Is it even possible I have deleted the default _trusted root certificates_ from inside _Keychain Access_, or why else can I not use the certificate anymore? Is there a way to reset or fix it?