There are two distinct methods for submitting emails over to the relay service. SMTP and API (HTTP).
Both SMTP and API connections will deliver outbound traffic in an identical manner. The way in which you connect to ReachMail's gateway is quite different, however, with each respective method.
Customers looking to connect appliances (scanners/faxes/voicemail-to-email) should use the SMTP Connection method. SMTP is likely the connection method of choice for non-technical users looking to hook up existing software to a relay for the purpose of delivering the email.
Senders building their own custom apps are advised to integrate with API connections. SMTP modules may be available for use but some may fall short when pushed to their limits. If you're not familiar with the chatty nature of the SMTP communication protocol some of its odd quirks can lead to unforeseen issues.
API connections offer a single request/response sequence that tends to make for a more robust and scalable process.