Re-added certificate leads to unknown authority
If you have in fact deleted a trusted root certificate, then you must


add it again
trust it again


In order to trust the certificate, open it by double-clicking on it in the keychain, open the Trust section, then set When using this certificate: Always trust.  

In order to make Mail accept the newly trusted certificate, it may be necessary to quit and re-start Mail.

If all fails, you could re-install your macOS over the existing installation which will most probably restore all pre-installed root certificates again.