Qmail is an MTA like Sendmail. If it also functions as an email client, I don't know anything about that. The email client software is the application that usually creates those files in the users home directory, where the MTA usually creates just an inbox in /var/spool/mail/username. If the email is retrieved remotely, it is usually handled by procmail or similar. If the email is retrieved locally, the email client pulls their messages from the spool into the users home folder.
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