Hello,
when I click on an emaillink (link mailto:) - then emClient creates an Email with my standard - account.
But I have more then one account - how can I quickly change the sender-address?
It should be like this:
It is just like that. There is a pull-down arrow next to your email address . You can select any email address you like, just as you show on your Ubuntu example.
Thank you! Now I have found the Arrow - BUT: see the Screenshot - ther was no arrow, because the window was to small 
On my Surface 4 it won’t let me make the window small enough to hide arroe.
Is there a way to set a preferred ‘from’ email address? I see my 2 addresses but the one I dont want to use is always the default.
Go to “Menu / Accounts”. Then click on your account you want default and click “Set as default” at the top of accounts.
Then when you compose and email that will be the default from email address.