Infrastructure Layer

The Relate.Smtp.Infrastructure project is the outermost layer of the backend, implementing the data access, background services, health checks, and telemetry that the Core layer defines as abstractions. This is where Entity Framework Core, PostgreSQL, BCrypt, DNS resolution, SMTP delivery, and OpenTelemetry live.

Responsibilities

• Data access -- EF Core AppDbContext with PostgreSQL, 9 repository implementations, Fluent API entity configurations, and database migrations
• Background services -- Outbound email delivery queue processor, background task queue for non-critical updates
• Authentication -- Shared protocol authenticator base class with rate limiting, caching, and BCrypt verification
• Email delivery -- SMTP delivery service with MX resolution and relay support
• Health checks -- Certificate expiry, connection pool, delivery queue, disk space, and memory monitoring
• Telemetry -- OpenTelemetry configuration with protocol-specific activity sources and metrics

Project Structure

Relate.Smtp.Infrastructure/
  DependencyInjection.cs              # Service registration
  Authentication/
    ProtocolAuthenticator.cs          # Shared auth base for POP3/IMAP/SMTP
  Data/
    AppDbContext.cs                    # EF Core DbContext
    Configurations/                   # 15 Fluent API entity configurations
    Migrations/                       # 4 database migrations
  Health/
    CertificateExpiryHealthCheck.cs
    ConnectionPoolHealthCheck.cs
    DeliveryQueueHealthCheck.cs
    DiskSpaceHealthCheck.cs
    MemoryHealthCheck.cs
  Repositories/                       # 9 repository implementations
    EmailRepository.cs
    UserRepository.cs
    SmtpApiKeyRepository.cs
    LabelRepository.cs
    EmailLabelRepository.cs
    EmailFilterRepository.cs
    UserPreferenceRepository.cs
    PushSubscriptionRepository.cs
    OutboundEmailRepository.cs
  Services/
    AuthenticationRateLimiter.cs
    BackgroundTaskQueue.cs
    DeliveryQueueProcessor.cs
    MxResolverService.cs
    SmtpDeliveryService.cs
    OutboundMailOptions.cs
    EmailNotificationService.cs
    IDeliveryNotificationService.cs
  Telemetry/
    TelemetryConfiguration.cs
    ProtocolMetrics.cs

Dependency Injection

DependencyInjection.cs provides a single extension method AddInfrastructure(connectionString) that registers all infrastructure services:

public static IServiceCollection AddInfrastructure(this IServiceCollection services, string connectionString)

Registered Services

Service	Lifetime	Description
AppDbContext	Scoped	EF Core database context
IEmailRepository	Scoped	Email data access
IUserRepository	Scoped	User account data access
ISmtpApiKeyRepository	Scoped	API key management
ILabelRepository	Scoped	Label CRUD
IEmailLabelRepository	Scoped	Email-label relationships
IEmailFilterRepository	Scoped	Filter rule management
IUserPreferenceRepository	Scoped	User preferences
IPushSubscriptionRepository	Scoped	Push notification subscriptions
IOutboundEmailRepository	Scoped	Outbound email management
BackgroundTaskQueue	Singleton	Non-critical async update queue
IAuthenticationRateLimiter	Singleton	Brute-force protection
ILookupClient	Singleton	DNS client (DnsClient.NET)
MxResolverService	Singleton	MX record resolution
SmtpDeliveryService	Scoped	Outbound email delivery
DeliveryQueueProcessor	Hosted	Background queue processor
BackgroundTaskQueueHostedService	Hosted	Background task consumer

Health Checks

The following health checks are registered:

Name	Tags	Description
database	(default)	EF Core database connectivity
disk-space	system	Available disk space monitoring
memory	system	Process memory usage monitoring
connection-pool	database	PostgreSQL connection pool utilization

Additional health checks (CertificateExpiryHealthCheck, DeliveryQueueHealthCheck) are registered by the individual protocol hosts and the API.

Connection String

The database connection is configured via:

ConnectionStrings__DefaultConnection=Host=localhost;Database=relatemail;Username=postgres;Password=...

Or in appsettings.json:

{
  "ConnectionStrings": {
    "DefaultConnection": "Host=localhost;Database=relatemail;..."
  }
}

Further Reading

• Data Access -- DbContext, entity configurations, migrations
• Repositories -- Repository implementation details
• Services -- Background services, delivery, rate limiting
• Health Checks -- Monitoring and alerting
• Telemetry -- OpenTelemetry integration