==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.9
- Deployed with iRedMail Easy or the downloadable installer? Downloadable
- Linux/BSD distribution name and version: Ubuntu 18.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MariaDB
- Web server (Apache or Nginx): nginx
- Manage mail accounts with iRedAdmin-Pro? no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I have a user a@abc.com that can send mail as an alias (from b@abc.com). How can I restrict all other users sending from b@abc.com. For example, I have a user c@abc.com that can also send from b@abc.com if it adds a new identity in Roundcube. So it turns out that every user in my system can send as b@abc.com. How can I allow only user a@abc.com to send as b@abc.com?

Thank you!