==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.9 MYSQL edition
- Deployed with iRedMail Easy or the downloadable installer? installer
- Linux/BSD distribution name and version: Ubuntu Ubuntu 18.04.1 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- 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.
====

Try to modify global sieve.  Before that, it seems to be me the file is intentionally protected.  May be it is maintained by the iredmail package/updates... I am not sure.

$ ls -la /var/vmail/sieve/dovecot.sieve
-r-x------ 1 vmail vmail 116 Jan  2 16:13 /var/vmail/sieve/dovecot.sieve*

What is the properly way to modify global sieve?  direct modifying the file and change back the original file mode? or add a custom file?  Thank you.