From what I understand, getting Siri to compose a new email requires proper integration with the eM Client App and at present this is not the case.
The Spark app supports this via shortcuts from what I can make out: " Yes, the Spark iOS app integrates with Siri through Siri Shortcuts, allowing you to perform actions like composing an email, searching for specific emails, or opening a folder with a voice command. You can create and customize these shortcuts within Spark’s settings"
Can / will eM Client support this as it’s a feature I used to use all the time with the default IOS ‘Mail’ App, that I no longer have installed since using the mobile version of eM Client.