How to add a domain to an existing LetsEncrypt SSL certificate
1) Connect to the server you are working with using a remote desktop program of your choice and launch IIS.
2) Click the dropdown arrow next to the server name on the left to expand it then click the dropdown arrow for Sites.
3) Right click SmarterMail and click Edit Bindings.
4) Click Add on the right side then enter the host name that you want to add to the SSL.
5) Navigate to Certify The Web and find the certificate you want to add the domain to.
6) Click Certificates on the right side of the screen, enter the domain name in the Add domains to certificate box, then click the plus sign icon next to the box.
7) Click Save at the top of the screen to save the changes.
8) Click Test in the upper right corner, verify the test is successful.
9) Click Request Certificate, verify the certificate was successfully issued.
10) Locate and open the Scripts folder, find the .bat file that corresponds with the certificate you are working with, right click the file then click Run as administrator.
11) Locate and open the Certificates folder, verify that the certificate was exported by checking the time on the certificate file to ensure it matches the current time.