- Added rate limiting for outbound email sends to prevent abuse, implemented in `internal/api/mail/sendguard`.
- Introduced idempotency key support for email sending to avoid duplicate submissions.
- Enhanced attachment handling with new limits and validation in `internal/api/mail/limits`.
- Updated outbox processing to include retry logic and circuit breaker for SMTP failures.
- Improved HTML sanitization for email content to enhance security.
- Added unit tests for new features, ensuring robust functionality and error handling.
- Updated configuration options in `.env.example` for new mail settings.
