← BackOAuth2-Based SMTP Infrastructure Migration
Enterprise-Scale, Security-Critical Backend System
I designed and led the migration of a server-wide SMTP infrastructure from legacy username/password authentication to OAuth2-based authentication in order to meet modern security and compliance requirements enforced by major email providers such as Google and Microsoft.
This infrastructure is a core dependency across multiple products, powering transactional emails, authentication flows, system notifications, and enterprise communication pipelines.
Context & Problem
Email delivery is a mission-critical component of the platform. The existing SMTP setup relied on basic authentication, which was deprecated by major providers due to security risks.
The challenge was not only technical but architectural: the SMTP system was deeply integrated across products, tenants, and enterprise configurations. Any disruption would directly impact user authentication, onboarding, and enterprise workflows.
My Role & Ownership
I owned the system end-to-end, from architecture and data modeling to implementation, rollout, and monitoring.
01Designing the OAuth2-based authentication architecture
02Refactoring internal SMTP abstractions within the PHP backend
03Coordinating rollout across multiple products and enterprise environments
04Ensuring backward compatibility and zero downtime during migration
Technical Architecture
Backend & Infrastructure
- Implemented OAuth2 SMTP authentication flows for Google and Microsoft providers
- Designed secure token lifecycle management (access tokens, refresh tokens, expiration handling)
- Integrated OAuth token refresh mechanisms into existing SMTP sending pipelines
- Built provider-agnostic abstractions to support future SMTP providers
PHP Backend & Internal Framework
- Extended the internal PHP framework to support OAuth2-based SMTP credentials
- Refactored existing SMTP services into reusable, testable components
- Introduced clear service boundaries between credential management, provider logic, and email dispatch
- Implemented feature flags and gradual rollout mechanisms at the service level
Data Modeling & Persistence
- Designed and migrated MySQL tables to store OAuth-related metadata securely
- Implemented encryption for sensitive fields (tokens, secrets)
- Ensured tenant-level isolation for enterprise accounts
- Built background jobs for token refresh and validation
Reliability & Observability
- Added structured logging and metrics around SMTP authentication and delivery
- Implemented failure detection and fallback strategies
- Introduced monitoring dashboards to track delivery success rates and authentication errors
Frontend & Admin Experience
- Built configuration flows for enterprise administrators to connect their email providers via OAuth2
- Implemented frontend flows with secure redirects and consent handling
- Used service workers to ensure resilience and retry mechanisms for configuration-related requests
- Provided clear UX feedback for token expiration, misconfiguration, and provider errors
Challenges Solved
→Migrating a deeply coupled, security-critical system without downtime
→Supporting multiple providers with different OAuth implementations
→Coordinating changes across backend services, databases, and frontend flows
→Maintaining uninterrupted email delivery during rollout
Impact
Zero
Downtime during migration
100%
Email delivery continuity
Enterprise
Compliance achieved
- Achieved zero-downtime migration across all affected products
- Ensured uninterrupted email delivery for hundreds of thousands of users
- Significantly improved platform security and enterprise compliance
- Established a future-proof SMTP architecture adaptable to provider changes
Outcome
The migration replaced legacy SMTP authentication with a modern, secure OAuth2-based infrastructure. The system is now more maintainable, extensible, and compliant, while preserving the reliability required by enterprise customers.