For my mbox file the following one-liner gives the same output as your program, except that the columns are reversed (as requested) and the data is unsorted:

    perl -lane '$h{"--$F[2]-$F[3]-$F[4]--$F[6]"}++ if /^From /;  /
                END{print $_, "\t", $h{$_} for keys %h}' SPAM-mbox
[download]

If you need sorted data you could use the system sort or add sort before keys above:

--
John.