Check the headers in the mail for any clues?
What is being used to send mail from your Wordpress app?

For an example, I tried registering another account on the site, the password arrived in spam.
Tried to use a plugin called Mail Bank to try and send test mail, failed too, or maybe I should use SMTP and if that, how do I get the specific postfix username p/w etc?

What are you exactly trying to send the mail as? Can you designate a from address?

Please check the headers in spam mail in your gmail inbox, it will tell you exactly why it was marked as SPAM ;0

The from address is admin@mydomain.com

in the header it says it was “78ae30bd8fcff3dfc2e6b15db8798ee4@mydomain.com”
is that just me being paranoid?

does your domain have SPF/ DKIM or DMARC set up?

Ok, so there is the error:
google.com: domain of transitioning dorpele10@gmail.com does not designate 45.55.235.233 as permitted sender

I have no idea, I just pointed everything to the right placed, how can I tell?

so it appear the mail is failing on gmails SPF records. SPF allows you to designate what IP address a domain can send from. In your case, a gmail account is sending from a non designated address which would mark it as SPAM, although i find that strange considering you said you were sending from admin@domain.com....