smtply

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SMTPly is email relay software enabling legacy devices to send messages via Microsoft 365 using OAuth2 authentication. Running as a local Windows service, it converts SMTP connections into Microsoft Graph API calls, ensuring data remains local without third-party servers. Its architecture includes an SMTP listener, relay queue with retry logic, and a Graph sender managing OAuth2 tokens for secure delivery. Security features include DPAPI encryption, certificate store integration, and health monitoring for certificate expiration alerts. A live mail tracker provides visibility into relayed messages, while usage reports summarize activity and errors. The software supports STARTTLS, implicit TLS, IP whitelisting, and sender domain filtering. Advanced editions offer multi-tenant routing, rate limiting, Prometheus metrics, and encrypted backups. Enterprise features include REST APIs, sandbox modes, outbound routing rules, and SIEM forwarding. Compliance tools ensure proof chains.