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The COUNTIF() function accepts only primitive level conditions such as:

=COUNTIF('mail-data'!D2:D1000, "6")

For more complex criteria we use virtual arrays:

=SUMPRODUCT(--('mail-data'!D2:D1000DATE(2005,12,1))*--('mail-data'!D2:D1000<DATE(2006,12,1)))

As you see we mutliply two arrays, which are based on D2:D100 yet are
virtual, producing a 0 or 1 in the positions of dates satisfying the
criterion. When a date is in dec2006 it satisfies both conditions,
hence the two 1's in this position multiplied give us 1 instead of 0,
which happens in all other cases. Summing the 1's gives you the count.

HTH
Kostis Vezerides