This document provides architectural and decision-making guidance for designing a scalable, reliable, and cost-efficient transactional email capability. It is targeted at CTOs and system architects evaluating different strategies for delivering critical system-generated communications such as password resets, billing notifications, or alerts.

Scope: Transactional email only. Marketing and campaign email are considered a separate function, typically handled by SaaS marketing platforms.

Current operational context: Approximately 1.2 million transactional emails per month. Reliability, deliverability, observability, and cost control are the main business drivers.

Reliability and Deliverability – The system must ensure that emails reach recipient inboxes consistently while maintaining domain reputation.

Cost Efficiency – Transactional emails are infrastructure-level communication; the chosen solution should provide predictable, low per-email cost.

Observability – Visibility into delivery performance, bounces, and complaints is necessary for operational assurance and reputation management.

Operational Overhead – Preference for managed or semi-managed components to reduce infrastructure maintenance and avoid direct IP management.

Security and Compliance – Alignment with modern authentication standards (SPF, DKIM, DMARC) and data protection policies.

Scalability – Capacity to handle growth in volume and burst loads without architectural rework.

SES provides a low-cost, highly reliable, managed email delivery service. Applications integrate directly using the SES API or SMTP interface. AWS manages IP reputation, scaling, and deliverability.

Strengths:

• Lowest operational cost (~$0.10 per 1,000 emails)

• High reliability and deliverability through AWS-managed IP pools

• Integrates natively with AWS security and monitoring (IAM, CloudWatch)

Limitations:

• Minimal observability; no native UI for message-level tracking

• Limited analytics; requires integration with monitoring or logging systems

Architectural fit: Best suited for organizations already operating within AWS with the capability to integrate basic metrics and feedback loops.

Applications send to an internal Postfix relay that then forwards messages to SES. This maintains current application SMTP configuration while offloading outbound delivery.

Strengths:

• Enables phased migration from legacy MTAs

• Allows custom routing and per-domain throttling

• Hides SES credentials from application layer

Limitations:

• Adds operational overhead (maintaining Postfix nodes)

• Redundant queuing and monitoring layers

Architectural fit: Transitional design for organizations migrating from self-hosted email infrastructure toward SES or similar managed delivery.

Postal acts as an intermediary service providing dashboards, message tracking, and analytics. SES remains the underlying delivery mechanism.

Strengths:

• Enhanced visibility and UI for message lifecycle

• Supports multi-tenant setups and central management of multiple domains

• Leverages SES deliverability and pricing

Limitations:

• Requires hosting and maintaining Postal’s components (application, database, queue)

• Slightly increased operational complexity

Architectural fit: Ideal for teams seeking richer observability and control while retaining SES’s cost advantages.

Commercial SaaS solutions combine delivery, analytics, dashboards, and support in one package.

Strengths:

• Excellent visibility, support, and compliance tooling

• Minimal operational responsibility

• High-quality deliverability and reputation management

Limitations:

• Costs typically 10× higher than SES (~$1,000–$1,500/month for 1.2M emails)

• Less infrastructure control

Architectural fit: Suited for organizations prioritizing convenience, comprehensive analytics, and minimal in-house operations.

Overall Recommendation: For large-scale transactional traffic, direct integration with AWS SES offers the most balanced combination of cost, reliability, and maintainability. Postal may be introduced as an optional visibility layer if richer operational insights are required without adopting high-cost SaaS solutions.

1. Decouple delivery from applications: Applications interface through APIs or a lightweight relay, avoiding direct MTA management.

2. Managed deliverability: Outsource IP reputation and scaling to managed providers such as SES.

3. Observability as an add-on layer: Integrate monitoring, metrics, and bounce handling through cloud-native or third-party tools rather than custom MTAs.

4. Security by configuration: Enforce domain authentication standards (SPF, DKIM, DMARC) and TLS transport for all communication.

5. Modular evolution: Allow the architecture to evolve from SES Direct → Postal/SES → SaaS provider without major refactor.

• Direct SES: ~USD 120/month at current scale. Cost scales linearly with volume.

• Postal Layer: Adds roughly USD 60–100/month in hosting and minimal administration.

• SaaS Alternatives: Between USD 1,000–1,500/month, justified primarily when advanced analytics and compliance reporting are mandatory.

CTOs should evaluate whether the added functionality of SaaS platforms offsets the 10× cost increase, especially when transactional communications are not business-critical revenue channels.

Adopt AWS SES Direct as the primary transactional email service. Introduce Postal later if internal visibility and multi-tenant management become priorities.

Maintain clear separation between transactional and marketing email systems to preserve deliverability and reputation. Use specialized marketing platforms for campaign activities.

The chosen architecture should prioritize:

• Simplicity and automation over manual MTA management.

• Observability through existing monitoring infrastructure.

• Compliance with authentication and privacy standards.