==== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.8
- Linux/BSD distribution name and version: CentOS 7.5
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro?: Yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Is it possible to configure iRedMail (or iRedAdmin-Pro or Roundcube) to stop users from forwarding email to a list of domains?

The problem is that some users forward all of their email to (for example) Gmail or Yahoo, and then when they receive spam in their free webmail account they click the "this is spam" button. Gmail and Yahoo respond by throttling mail from our server, both forwarded mail and mail sent directly to users on their system. Users don't realise that they are hurting themselves, as then all of their email is delayed or bounced. I'm thinking of mounting an educational programme to encourage users not to do this, with the end goal that at some point I might simply not allow users to forward their email to gmail.com, yahoo.com, hotmail.com, outlook.com, etc.