Here's the diff:
<img id="logo" src="https://singly.com/images/singly-logo_300.png"/>
When I change to using an embedded image all the styling is lost. I checked the file path (tried a view variants).
I'm not sure if thats a Nodemailer issue as Nodemailer does not touch any stylings of the e-mail. Maybe the client that shows the message somehow messes something up, when embedded images are used.