What is the primary protocol used for SOAP web services integration in Mule?

The primary protocol used for SOAP web services integration in Mule is HTTP. SOAP, which stands for Simple Object Access Protocol, is a protocol that relies on XML as its message format and is typically used for exchanging structured information in web services. HTTP serves as the transport layer for SOAP messages, allowing for seamless communication between client and server.

SOAP messages are encapsulated within HTTP requests, and the response is also delivered over HTTP. This makes HTTP crucial for the functioning of SOAP web services, as it handles the request-response model necessary for the exchange of data.

Other options like SMTP, FTP, and MQTT serve different purposes. SMTP is primarily used for email transmission; FTP is focused on file transfer; and MQTT is designed for lightweight messaging, often in IoT scenarios. While these protocols are integral to their respective domains, they do not play a role in the SOAP web services' architecture within MuleSoft. Thus, HTTP is the clear and correct choice for SOAP web service integration.