Opaque error with self-hosted Sourcegraph and Google Workspace SMTP relay config

Question

Finally deployed a self-hosted Sourcegraph, v3.39.1. I'm running on Google Compute Engine on a Container Optimized OS VM. Got everything working except I'm having difficulty getting SMTP set up though Google Workspace's smtp-relay. When I run a sendTestEmail command, I get back an opaque error:

{
  "data": {
    "sendTestEmail": "Failed to send test email: EOF"
  }
}

Here is a redacted snippet of my config:

  "email.address": "[email protected]",
  "email.smtp": {
    "authentication": "PLAIN",
    "domain": "mydomain.com",
    "username": "[email protected]",
    "password": "REDACTED",
    "host": "smtp-relay.gmail.com",
    "port": 587
  }

For a quick smoke test, I ran netcat (nc smtp-relay.gmail.com 587) from the container and did sent EHLO mydomain.com. The smtp-relay responded with a "at your service" so I assume that worked. Any one have any other tips? Any logs I can check? (I didn't see anything obvious, but I'm new to Sourcegraph)