Scaling Transactional Email: Architectural Choices for Modern Tech Teams

Every fast-growing tech company reaches the point where sending emails becomes an infrastructure decision, not just a developer feature. What starts as a few password resets and alert emails per day quickly grows into millions of messages per month.

At that scale, questions arise: Should we run our own mail server? Use AWS SES? Adopt a SaaS like Mailgun? Or something in between?

In this article, we’ll explore the reasoning process behind designing a transactional email architecture that balances reliability, deliverability, visibility, and cost — without adding unnecessary operational weight.

“Email infrastructure decisions are not about sending messages — they’re about scaling trust, reliability, and deliverability.”

A transactional email system automates the sending of system-generated communications — like password resets, receipts, and alert notifications. Unlike marketing email, which targets lists of users, transactional email is event-driven and personal.

At its best, transactional email infrastructure is invisible — quietly handling millions of messages while maintaining brand reputation and compliance.

As organizations scale, email stops being a developer convenience and becomes a reliability concern.

• Volume grows beyond what internal mail servers can handle efficiently.

• Deliverability becomes unpredictable due to reputation issues or IP throttling.

• Visibility into failures, bounces, and complaints becomes critical.

• Cost escalates when moving to SaaS without careful planning.

The right architecture must balance these trade-offs — offering reliability and observability at sustainable cost.

“Email is the heartbeat of transactional trust — but few treat it with the same rigor as their API uptime.” — Cloud Architecture Lead, FinTech Sector

Amazon Simple Email Service (SES) is the most cost-effective way to send at scale. At $0.10 per 1,000 emails, it’s nearly impossible to beat in raw cost-efficiency.

SES handles IP reputation, DKIM signing, scaling, and compliance. It’s ideal for companies already within the AWS ecosystem and comfortable with infrastructure automation.

• Extremely low cost

• Highly reliable and scalable

• Strong AWS-native security and IAM integration

• Limited analytics or message-level visibility

• Requires custom metrics integration (CloudWatch or Prometheus)

For teams migrating from legacy Postfix or in-house mail relays, a Postfix → SES architecture provides an easy bridge.

Applications send to an internal relay (Postfix), which queues and forwards messages to SES. This preserves existing SMTP flows while outsourcing actual delivery.

• Simplifies migration from legacy MTAs

• Enables per-domain throttling and routing

• Keeps credentials isolated from application layer

• Adds operational overhead for maintaining Postfix

• Slightly redundant queuing and monitoring

Postal, an open-source mail platform, adds a UI and analytics layer on top of SES. It centralizes logging, bounce management, and message tracking while using SES for delivery.

This hybrid model offers much of the visibility of SaaS without its price tag.

• Clear dashboards and message lifecycle visibility

• Multi-domain management from a single pane

• Uses SES for reliable, cost-effective delivery

• Requires operating Postal infrastructure (app + DB + queue)

• Slightly higher operational complexity than SES alone

“Postal + SES gives you SaaS-level insight without SaaS-level cost.”

SaaS providers combine delivery, reputation management, analytics, and compliance into one managed service. They’re plug-and-play, but they come at a premium — typically 10× the cost of SES.

• Rich analytics and logs

• Dedicated support and compliance handling

• Zero infrastructure to maintain

• High recurring cost

• Limited flexibility and dependency on vendor stack

• Running self-hosted MTAs introduces IP warm-up, spam risks, and maintenance pain.

• Jumping to SaaS too early can multiply costs without improving delivery.

• Neglecting observability means flying blind on failures.

The right design separates delivery from monitoring — SES (delivery) plus your chosen visibility layer — providing control and clarity at predictable cost.

Quote: “If you can’t see where your messages go, you’re not in control of your communications stack.”

1. Separate transactional from marketing traffic. They differ in volume, purpose, and compliance requirements. Mixing them risks domain reputation.

2. Authenticate everything. Always use SPF, DKIM, and DMARC with proper alignment.

3. Monitor reputation metrics. Track bounce and complaint rates, especially with new domains.

4. Automate feedback processing. Use SNS or webhooks to handle bounces programmatically.

5. Design for evolution. Start simple (SES direct), then add Postal or SaaS layers if visibility or compliance needs grow.

“Start with SES for reliability, add Postal for insight, and move to SaaS only when compliance justifies the cost.”

These benefits align well with the scaling patterns of SaaS, fintech, and marketplace startups moving toward cloud-native, API-driven infrastructure.

• Start with SES Direct if you value simplicity and cost efficiency.

• Layer Postal when teams need dashboards, multi-domain visibility, or delegated control.

• Consider SaaS only when cost is secondary to compliance reporting or enterprise integration.

Transactional email isn’t just infrastructure — it’s part of your trust fabric. Treat it with the same architectural discipline as your API or payment stack.