Why do some companies build their own mail servers?

Despite the convenience of ESPs, some organizations choose to build and operate their own mail infrastructure.

Reasons for self-hosting:

Control: Complete authority over configuration, sending behavior, and data. No reliance on ESP policies or platform changes.

Cost at scale: Very high volumes (hundreds of millions monthly) may be cheaper to self-host than pay per-message ESP fees.

Data residency: Strict requirements about where data resides and who can access it. Full control over geographic placement.

Customization: Unique routing logic, integration requirements, or sending patterns that ESPs can't accommodate.

Independence: No vendor lock-in, no platform risk, no third-party in the sending path.

Logging and auditing: Complete access to every detail of every transaction for compliance or analysis.

Who typically self-hosts:

Large tech companies with engineering resources

Organizations with strict compliance requirements

Companies sending at massive scale

Businesses with unique technical requirements

Anti-spam and security vendors (for obvious reasons)

Some shipping companies own their fleets; others charter vessels. The right choice depends on volume, expertise, and strategic priorities.