SOGoMailComposeWindow = inline;

As you said, this will only apply to new users. For an existing user, you need to use sogo-tool.

@francis
Thank you very much :)

As I'm not able to find this option here:
https://www.sogo.nu/files/docs/SOGoInstallationGuide.html

Is there a place with a more complete documentation?

I've update the installation guide with this parameter.

You can always refer to SOGoDefaults.plist to see the default values, but it doesn't include the optional parameters (such as SOGoMailComposeWindow).