This presents as a weird issue where you can log into the the ecp but then get a 503 error, you can’t use the Exchange management shell and the users clients are showing a certificate error but the certificate is valid.
I recently renewed a certificate on a Exchange server and 2 weeks later after a reboot it dropped the certificate from it’s bindings.
- Launch IIS Manager
- Expand “Sites” under the server with this issue
- Click on “Exchange Back End”
- In the actions pane to the right, click on “Bindings”
- Select the https type and click on “Edit”
- Select the appropriate certificate and click on Ok.