What is HA (High Availability) in email delivery?

High Availability (HA) describes systems designed to remain operational despite component failures. For email infrastructure, HA means sending continues even when parts of the system fail.

HA design principles:

No single points of failure: Every critical component has redundancy. If one MTA fails, others continue; if one database server fails, replicas take over.

Automatic failover: Systems detect failures and reroute traffic without manual intervention. Health checks identify problems; failover happens in seconds or minutes.

Graceful degradation: Partial failures reduce capacity but don't cause complete outages. Fewer available servers means slower sending, not stopped sending.

HA metrics:

Uptime percentage: 99.9% (8.76 hours downtime/year), 99.99% (52.6 minutes/year), 99.999% (5.26 minutes/year)

MTBF: Mean Time Between Failures

MTTR: Mean Time To Recovery

HA in practice for ESPs:

Clustered MTAs with load balancing

Database replication with automatic failover

Multiple data centers with traffic distribution

Redundant network connectivity

Monitoring and alerting for rapid response

Enterprise ESPs typically commit to 99.9% or higher uptime SLAs. Self-hosted systems require careful architecture to achieve similar availability.