What’s the difference between horizontal and vertical scaling?

Vertical scaling (scaling up) means adding more resources to a single server: more CPU, RAM, faster storage. Horizontal scaling (scaling out) means adding more servers to distribute the load.

Vertical scaling:

Simpler architecture (one server to manage)

No distribution complexity

Limited by maximum hardware capabilities

Single point of failure

Often more expensive at high end

Horizontal scaling:

Theoretically unlimited capacity (add more servers)

Built-in redundancy (servers can fail without total outage)

More complex architecture (coordination, distribution)

Often more cost-effective at scale

Requires distributed systems design

Email infrastructure typically uses horizontal scaling:

MTA clusters with many servers

Distributed queues across machines

Sharded databases for subscriber data

Multiple data centers for geographic distribution

The nature of email (high volume, many independent messages, need for redundancy) suits horizontal scaling. You want sending to continue if a server fails, and you need capacity that exceeds single-server limits.

Some components may still scale vertically where it makes sense (database primary servers, for instance), but the overall architecture is horizontal.