One of the most infamous questions in all of history. Of course, the obvious answer is "wherever you put them."
Usually when one (referring to qmail) says "look at your logs" they are referring to the qmail-send logs, since only qmail-send really logs anything. They do not know where your logs are, because (well, maybe they did) they did not set up your installation, You did. Therefore, you have to answer that question for yourself, because only you can know. Why? qmail-send sends out its logging information on stdout. You take that logging information and place it somewhere, generally through the use of a daemontools logging service, or via a logging program given to qmail-start on the command line, or by piping the output to another program to handle the logging.
man qmail-send
man qmail-start
man qmail-log
Jeremy Kitchen - kitchen at scriptkitchen dot com - 1/13/2004