Try this instead:
perl /usr/bin/spamfilter-stats-7.pl /var/log/qpsmtpd/*
The log files obtain a ".s" extension if they rotate without errors. If there is any sort of error, they get a ".u" extension, and if you haven't received enough email to generate at least 5MB of log data there will be no files of either type.
Also, for the mailstats to work correctly you have to have your qpsmtpd log level set to 8. You can check this with:
config getprop qpsmtpd LogLevel
LogLevel was set to a lower value in one of the SME 7 RC versions, then set back to 8 when SME 7 final was released. If you started with an RC version and upgraded it might have stayed at the lower level.