What is a common response from an SMTP server to an invalid command?

When an SMTP server receives an invalid command, it typically responds with a syntax error message. This indicates that the command received does not conform to the standards of the SMTP protocol. A syntax error is a way for the server to inform the client that the request made does not follow the proper format or that the command is unrecognized. This response is crucial because it helps maintain proper communication between email clients and servers, ensuring that commands are understood and processed correctly.

In contrast to other options, a server timeout typically refers to a situation where the server fails to respond within a designated period. Connection lost implies a break in communication, which is not directly related to command validity. Command acknowledged suggests that the command was understood and accepted, which is not the case for an invalid command. The clear response of a syntax error thus highlights the importance of adhering to correct format and protocol in SMTP communications.