Sendmail can show you the outgoing queue (messages submitted but not yet sent) but only its log file will receive already sent and received messages. Making matters more complex, you might have a different MTA than stock Sendmail, in which case the options and the format and location of the log files might be different. Before going into more detail, perhaps it would be best if you could describe what you are trying to accomplish, and perhaps post information about which version of Sendmail and which Unix distro you are using.
Sendmail is the tool for routing mail on the Internet (an MTA). It's not concerned with the local folder structure of a user's mailbox. There are about half a dozen MUA folder formats, but the two most popular are the traditionaly Berkeley mbox format, and the newer maildir format. See also
Procmail FAQ
Berkeley mbox format is a single file; a folder structure is simulated by having multiple such files, each containing one "folder" of messages. Typically these are in a directory such as $HOME/Mail but there is no particular technical need for this to be so. Internally, each file has the messages as simple chunks of text, each new message beginning with the five characters "From " at the beginning of line, followed by the sender's email address and (usually) a time stamp. This separator line is technically not part of the message itself.
Maildir is a directory structure, again typically but not necessarily in a directory such as $HOME/Mail. Each subdirectory with the directories "new", "cur", and "tmp", is a folder; new messages are created as individual files in the "new" directory, and transition through "tmp" to "cur" when they are received / read. In practice, you will rarely see any messages in the "tmp" directory.
On a modern system which is connected to the Internet, Sendmail will not queue an outgoing message for any substantial amount of time unless there is a delivery problem. Intermittently-connected dial-up systems are often configured so that messages are queued up and sent only when the system is connected to the Internet. On such a system, your question would make more sense, but again, it would help to understand what you are trying to accomplish before explaining this in more detail.
