IP pool management for high-volume sending

At volumes above 100,000 emails per day, a single sending IP becomes both a performance bottleneck and a single point of reputation failure. A block on one IP stops all sending until it is manually removed and replaced — a process that can take hours. IP pool management distributes sending load across multiple IPs and allows the system to automatically exclude a blocked or high-bounce IP and continue sending through healthy ones, without downtime or engineering intervention. Reputation-weighted or least-used selection also prevents one IP from absorbing a disproportionate share of sends while warm-up periods complete on others.

Store sending IPs in the database with per-IP health and warm-up metadata, then select from the pool based on health and current load:

export async function selectSendingIp(): Promise<string> {
  const candidates = await db.sendingIp.findMany({
    where: { active: true },
    orderBy: { sentToday: 'asc' }  // Prefer least-used
  })

  if (candidates.length === 0) {
    throw new Error('No available sending IPs')
  }

  // Exclude IPs with bounce rates above 5% — they are reputation-damaged
  const healthy = candidates.filter(ip => ip.hourlyBounceRate < 0.05)

  // Fall back to any active IP if all are bouncing above threshold
  return (healthy[0] ?? candidates[0]).ipAddress
}

Add an active boolean column to the SendingIp table so a damaged IP can be removed from rotation by setting active = false — no code deployment required. Pair with the warmup schedule check from deliverability-engineering.warmup-reputation.automated-warmup-schedule so IPs in early warm-up are not selected for full-volume sends.