Email settings 502

Hi all
I’ve been all over the forums and github issue boards trying to understand what my droplet might be missing in order to set up my Kanboard email for registering, email invitations and notifications.

I’m a complete beginner when it comes to email setup, just started learning, but I feel like I’ve hit a roadblock.
I set up a simple, send only postfix smtp server on my droplet based on this guide:

When I’m trying to add an outgoing email in email settings I get a 502 error from Nginx.

I feel I’m missing some crucial steps here due to huge knowledge gaps. It would be awesome if someone could at least point me in the right direction!