The following are mail servers that work in C/S mode except? A. Sending agent B. Sending services C. Receiving agent D. Receiving agent

The question seems to be incomplete or incorrectly formatted. The options provided are not all different: "Receiving agent" is repeated twice. Also, the term "C/S mode" refers to Client/Server mode, which is a model of interaction in networked systems.

However, based on the options provided, we can infer that the question is asking which of the following is not a type of mail server.

A. Sending agent B. Sending services C. Receiving agent D. Receiving agent

In this case, the answer would be B. Sending services. This is because "sending services" is not a type of mail server, but rather a function that a mail server might perform. On the other hand, a sending agent and a receiving agent could be considered types of mail servers, as they refer to the server's role in either sending or receiving mail.