==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.3.1 MARIADB edition.
- Deployed with iRedMail Easy or the downloadable installer? downloadable installer
- Linux/BSD distribution name and version: CentOS Linux release 8.2.2004 (Core)
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

When i try to send an email using my python code that's using smtplib, email was sent successfully but CC header is ignored by postfix for some reason.
When i receive an email, i see the recipient in CC field and i can use "Reply All". But CC recipient hasn't received that email and Postfix logs do not show that email was sent to CC. If i use "Reply All", CC will then receive my reply.

This is the header that is being formed by Python.

Content-Type: multipart/mixed; boundary="===============0783044642835671299=="
MIME-Version: 1.0
From: admin@example.com
To: someemail@other.com
Cc: cc-someemail@other.com
Subject: Bill

--===============0783044642835671299==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

Auto generated. Bill is: 1234
--===============0783044642835671299==
Content-Type: application/octet-stream
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename= Racun.pdf

JVBERi0xLjQKJeLjz9MKMyAwIG9iago8PC9JbnRlbnQvUGVyY2VwdHVhbC9EZWNvZGVQYXJtczw8
...

Any ideas why would this be ignored? Much appreciated!