What is hybrid sending (ESP + custom MTA)?

Hybrid sending combines commercial ESP services with self-hosted mail infrastructure. Different email streams route through whichever system best serves their needs.

Common hybrid configurations:

Marketing via ESP, transactional via custom MTA: ESP provides campaign tools; custom MTA handles high-volume transactional with full control

Overflow handling: Custom MTA for baseline volume; ESP for peak capacity

Geographic routing: Regional MTAs for data residency; ESP for other regions

Specialized streams: ESP for marketing; dedicated MTA for critical alerts

Benefits of hybrid:

Best of both worlds: ESP features + custom control

Cost optimization for different volume types

Redundancy across platforms

Compliance flexibility (data residency, logging)

Challenges:

More complex architecture to manage

Split visibility into deliverability metrics

Authentication must work for both paths

Subscriber data may be in multiple systems

Requires expertise in both ESP and MTA management

Hybrid setups are common for larger organizations with diverse email needs and engineering resources to manage complexity.