While installing certificate I encountered certificate invalid in Exchange 2013. I tried couple of times but with  no luck.

Checked if root certificate is missing from the Trusted Root Certificate Authorities on the Exchange server via MMC console. Loo and behold – wasn’t there.

To resolve this issue,
1.  go to web enrollment http://caserver.abc.com/certsrv and Download CA Certificate.
2. Open MMC on Exchange Server and add certificate snap-in . Navigate to the path Console Root->Certificates->Trusted Root Certificate Authorities-Choose Import option by right clicking on the All Task.
3. Browse the certificate that you downloaded earlier. Place the Certificate in the Trusted Root Certification Authorities Store.
4. Refresh the Exchange ECP. Certificate should show valid.