Specifying Custom Headers

Use the headers property to specify any custom headers (note that these can also be set globally per the API specification:

const msg = {
  to: 'recipient@example.org',
  from: 'sender@example.org',
  subject: 'Hello custom header',
  html: '<p>Some email content</p>',
  headers: {
    'X-CustomHeader': 'Custom header value',
  },
};