To install a custom SSL/TLS certificate for the Courier-IMAP (IMAP/POP3) mail server on a Plesk server:

1. Log in to a Plesk server through SSH as a root user.

2. Download one or more SSL/TLS certificate files that you want to install.

   Note: IMAP and POP3 each require separate certificate files, but both files can contain the same certificate.

3. Make a backup copy of the existing default SSL/TLS certificate for the Courier-IMAP mail server.

   For example for RedHat or Fedora operating systems, you need to back up the following default SSL/TLS certificate files:

   • /usr/share/imapd.pem - the certificate enables secure data transfers through the IMAP protocol.

   • /usr/share/pop3d.pem - the certificate enables secure data transfers through the POP3 protocol.

     Note: For other operating systems, the default certificate file locations may be different.

4. Open a default certificate file using your favorite text editor and replace the contents of the file, with the content of the SSL/TLS certificate file that you want to install.

   For example, the content to be copied from a custom SSL/TLS certificate and pasted in lieu of a default certificate file body should look like this:

   -----BEGIN CERTIFICATE-----
   MIIB8TCCAZsCBEUpHKkwDQYJKoZIhvcNAQEEBQAwgYExCzAJBgNVBAYTAlJPMQww
   ............
   ............
   eNpAIeF34UctLcHkZJGIK6b9Gktm
   -----END CERTIFICATE-----
   
   -----BEGIN RSA PRIVATE KEY-----
   MIICXgIBAAKBgQDv6i/mxtS2B2PjShArtOAmdRoEcCWa/LH1GcrbW14zdbmIqrxb
   ..........
   ..........
   faXRHcG37TkvglUZ3wgy6eKuyrDi5gkwV8WAuaoNct5j5w==
   -----END RSA PRIVATE KEY-----
   

5. Save and close the file.

6. To finish the certificate installation, restart Courier-IMAP.