Emailing customer from admin customers area, what account from?

I'm using 4.60.6 and I needed to contact a customer. I searched for them in the Customers area of the Admin and clicked EMAIL at the top right to email them. I have no idea what email account is used for this email, I assume the "default" account? Or what template, if any is used. Can someone inform me what is used both in email and template for an email generated from the Customers area of the Admin?

Yes, "default email account".
No message template.  The Body is sent as-is.  You can enter HTML.  If you "Edit" the message in the Message Queue, you can see (edit) the HTML in the "Editor".