How to limit access to a webmail in Plesk to a specific IP?

Question

How to restrict access to webmail.example.com for all IP except one? 

Answer

1. Connect to the server via SSH

2. Create a custom template directory if it doesn't exist.

   # mkdir -p /usr/local/psa/admin/conf/templates/custom/webmail

3. Copy the Roundcube template

   # cp -a /usr/local/psa/admin/conf/templates/default/webmail/nginxWebmailPartial.php /usr/local/psa/admin/conf/templates/custom/webmail/nginxWebmailPartial.php

4. Open the template /usr/local/psa/admin/conf/templates/custom/webmail/nginxWebmailPartial.php and add inside the block "server {}":

   CONFIG_TEXT: allow 203.0.113.10;
   allow 198.51.100.20;
   allow 192.168.10.0/24;
   allow 2001:db8:1234::/64;
   deny all;

5. Rebuild configurations:

   # /usr/local/psa/admin/bin/httpdmng --reconfigure-all