220 Features: a . QUIT 221 Bye.QUIT parameters are prohibited. Some servers will reject any QUIT request with a nonempty parameter.
If the server accepts the QUIT request (required code 221), it then closes the connection without reading further requests. The server also stops listening for data connections and drops any data connections already accepted.
There are a few broken TCP implementations, such as MacTCP 2.0.6, that fail to acknowledge TCP FINs after a local close. If the client is running on such a host, it shouldn't close the connection until after it sends QUIT and sees the server close the connection; otherwise the server will waste time repeatedly transmitting the FIN until it times out.
A high-quality server will close the connection if it runs out of memory or other local resources, or if the client is inactive for a while. Many servers will send a response with code 421 before closing the connection; after the client sends its next request, it will receive the 421 response and treat it as temporary rejection. Other servers do not bother sending 421.