The Transactional Messaging API is the correct recommendation because it uniquely satisfies both explicit requirements simultaneously: the message is dispatched as fast as possible through a direct, synchronous API call initiated the moment the password-reset request occurs on the web server, and critically, the API call itself returns a real-time send-status response back to the calling web server, satisfying the requirement that ' their web server should receive a notification when the email is sent ' — a synchronous confirmation mechanism that a standard Triggered Send Definition (C) does not provide in the same direct, immediate way, since Triggered Sends fire based on a configured trigger without returning an inline API response to the calling system. A Multi-Step journey with an API Event entry source (B) introduces journey-evaluation timing overhead and, like Triggered Send Definitions, does not natively provide the synchronous, per-call send-status confirmation back to the web server that the Transactional Messaging API ' s request/response model delivers. An Email Send Activity (D) in Automation Studio is a batch, scheduled mechanism entirely unsuited to the fast, individual, event-driven, confirmation-requiring nature of this password-reset use case. Reference: Transactional Messaging API.