It seems to me that the operation doesn't give any information to any participant. Closing the TCP connection by the client can be detected by the server just as well. And the client doesn't need any kind of acknowledgement after deciding to disconnect from the server and calling QUIT.
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Why does nearly every Windows application have a File > Quit menu option, when you can just close the window? Because that's how they made it. – cHao Jan 04 '14 at 20:36
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The only reliable way to make sure everything is completed as intended is to receive an explicit message indicating that. – Roman R. Jan 04 '14 at 20:39
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@RomanR. Isn't the acknowledgement of the last command preceding QUIT sufficient? I thought that SMTP doesn't defer processing of commands and after a command is acknowledged, it's known to be processed. – Petr Jan 04 '14 at 20:42
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It's just a part of graceful termination. Seeing closed connection there is ambiguity on server as for whether the connection is lost or closed intentionally by remote party it is talking to. `QUIT` at any time says there are no networking issues involved. Of course server treats those similarly freeing resources associated with session. – Roman R. Jan 04 '14 at 20:52