@emotion Yay, their email response gave me an idea to use "name" option in nodemail configuration, and it worked!
I had the same problem, emails had been correctly sent but a week ago or so they stopped, in logs i saw that 421, "4.7.0" error.
Here is what i use as transport configuration now

{
    "name": "your-domain-name.com",
    "host" : "smtp-relay.gmail.com",
    "port" : 465,
    "secure": true,
    "pool": true
  },

Thank you!