Wietse Venema writes:

Say what?

http://www.qmail.org/qmail-manual-html/man5/maildir.html:

A program delivers a mail message in six steps. First, it chdir()s to the maildir directory. Second, it stat()s the name tmp/time.pid.host, where time is the number of seconds since the beginning of 1970 GMT, pid is the program's pro- cess ID, and host is the host name. Third, if stat() returned anything other than ENOENT, the program sleeps for two seconds, updates time, and tries the stat() again, a limited number of times. Fourth, the program creates tmp/time.pid.host. Fifth, the program NFS-writes the mes- sage to the file. Sixth, the program link()s the file to new/time.pid.host. At that instant the message has been successfully delivered.

Of course, using pid is no longer sufficient. But that doesn't change the fact that more stuff is, in fact, using pid than not. That's because pid was, in fact, explicitly specified in the beginning. Evolutionary-necessitated modifications to the above need to take that into account.