Marketing and transactional email separated across IPs or subdomains

Marketing email and transactional email have fundamentally different engagement profiles. Marketing email generates higher complaint rates, lower open rates, and more unsubscribes than password resets, receipts, and account alerts. When they share an IP and sending domain, the reputation damage from a poorly-performing campaign delays or blocks the delivery of time-sensitive transactional messages — password resets that users are actively waiting for, order confirmations that trigger support tickets when delayed. Traffic separation is the architectural guarantee that a bad marketing campaign cannot block the transactional email infrastructure.

Define separate sending configurations for marketing and transactional email and route explicitly at the send-call level. Both configs should be authenticated independently:

// In src/lib/email/config.ts
const SENDING_CONFIG = {
  transactional: {
    from: 'noreply@tx.company.com',
    smtpHost: process.env.SMTP_HOST_TRANSACTIONAL,
    ipPool: process.env.IP_POOL_TRANSACTIONAL,
  },
  marketing: {
    from: 'news@mail.company.com',
    smtpHost: process.env.SMTP_HOST_MARKETING,
    ipPool: process.env.IP_POOL_MARKETING,
  }
} as const

export function getSendingConfig(type: 'transactional' | 'marketing') {
  return SENDING_CONFIG[type]
}

Call getSendingConfig('transactional') for password resets, receipts, and alert emails. Call getSendingConfig('marketing') for newsletters, drip campaigns, and promotions. Authenticate tx.company.com and mail.company.com with independent SPF, DKIM, and DMARC records.