I've found the cause, the Wordpress SSO plugin was causing issues.
Email configuration (mailgun) issues
There is a mismatch between what I've configured and what the users receive via email.
From email address: "email@example.com", from email name: "Classy cars".
It displays "forum" in the email client.
There are two brackets  which are in the subject and I don't know what what represent and how to remove/change them.
What have you configured as the site name?
Yeah, site title, I think there's a browser title thing as well
@PitaJ those are both set.
Where else is the email message customized?
I've mixed a couple of themes and maybe these are wrong.
@PitaJ and who else may encounter this issue, here is how I solved it.
The root problem was the image format used for "Site Logo" in general/settings.
I've uploaded a png file and the information was not saved in the database, althoughthe fields remained filled across the sessions.
Once I've changed the site logo with a jpg file, the emails were properly formatted too.