The Transactional Messaging API is Salesforce ' s purpose-built, API-driven mechanism for sending exactly this class of message — a one-off, highly time-sensitive, security-relevant notification (new-IP login alert) that must be dispatched with minimal latency directly from an external system ' s real-time event trigger, without waiting on any batch, scheduled, or journey-evaluation cycle. Because the request is initiated synchronously via a direct API call at the exact moment the security event (new IP login) occurs, and the response includes send-status confirmation, it delivers the fastest possible deployment path among the available options while also supporting the dynamic, per-message content (the specific login IP) that needs to be included. Injecting the subscriber into a Journey (A) introduces journey entry-evaluation timing and processing overhead that, while often fast, is not as directly immediate as a synchronous transactional API call purpose-built for exactly this kind of urgent, individual, security-triggered message. ' User Initiated Emails Interaction ' (B) and ' Triggered Emails Interaction ' (D) are not standard, precisely-named Marketing Cloud features matching this real-time, API-driven transactional use case as directly as the Transactional Messaging API. Reference: Transactional Messaging API, Security/Fraud Alert Messaging Patterns.