await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'Email with Custom Headers',
  html: '<p>Hello!</p>',
  headers: {
    'X-Campaign-ID': 'summer-sale-2024',
    'X-Customer-ID': 'cust_123',
    'X-Priority': 'high'
  }
});

await lettr.emails.send({
  from: 'noreply@example.com',
  to: ['recipient@example.com'],
  subject: 'Support Ticket Update',
  html: '<p>Your ticket has been updated.</p>',
  replyTo: 'support@example.com'
  // Or via headers:
  // headers: { 'Reply-To': 'support@example.com' }
});

await lettr.emails.send({
  from: 'newsletter@example.com',
  to: ['recipient@example.com'],
  subject: 'Weekly Newsletter',
  html: '<p>Newsletter content...</p>',
  headers: {
    'List-Unsubscribe': '<https://example.com/unsubscribe?email={{email}}>, <mailto:unsubscribe@example.com>',
    'List-Unsubscribe-Post': 'List-Unsubscribe=One-Click'
  }
});

// Original email
const original = await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'Initial Message',
  html: '<p>Hello!</p>'
});

// Reply (will be threaded)
await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'Re: Initial Message',
  html: '<p>Following up...</p>',
  headers: {
    'In-Reply-To': original.messageId,
    'References': original.messageId
  }
});

await lettr.emails.send({
  from: 'system@example.com',
  to: ['recipient@example.com'],
  subject: 'Automated Notification',
  html: '<p>System notification...</p>',
  headers: {
    'X-Auto-Response-Suppress': 'All',
    'Auto-Submitted': 'auto-generated'
  }
});