Yes Nuxwin, I know about reverse DNS / name mail server. It's all OK from this side, only issue is that postfix must use the MX IP address as a sent server.
I saw the french tutorial and I understand what it's all about but the idea is to integrate somehow in the i-mscp engine not to add manually the ip and hello name for each domain.
If we figure this out somehow it would be a great idea to implement it in the next version of i-mscp.
It's exactly for why I proposed such configuration as addon.
You can open a ticket if you want.
Thank you for using i-MSCP.