How many sending domains should I use?

The right number depends on your email program's complexity. At minimum, consider separating transactional and marketing streams. Transactional emails (receipts, password resets, shipping notifications) are critical; you don't want marketing reputation issues affecting their delivery.

Larger organizations often add more segmentation: corporate communications, product notifications, different brand lines, or regional domains. Each separation provides reputation isolation but adds DNS management overhead.

Don't over-segment. Every domain needs sufficient volume to build and maintain reputation. Ten domains each sending 1,000 emails monthly perform worse than two domains sending 5,000 each. Balance isolation benefits against the volume required for healthy reputation.

A merchant fleet needs enough cargo for each ship. Too many vessels chasing too little trade leaves all of them underprovisioned.