The error mysteriously disappeared and was replaced by an authentication error. I managed to have my Postmark account unlocked (btw, their support team was way much better than that of Sendgird, which still hasn't responded for 2 days since I requested till I am typing right now) and found that to use Postmark in Nodemailer you have to set the server api token as both the username and the password. This is quite counter intuitive, in my opinion.
And I finally figured out that, the test email button works on your last saved settings, not what you are editing in the input boxes right now.
So now Postmark is working. Thank God.