Assuming the total number of emails in A1 and the beginning date for the
period in B1:
=A1/(YEAR(NOW())-YEAR(B1)+1) gives average per year
=A1/INT(NOW() - B1) gives average per day
=A1/INT(NOW() - B1)*7 gives average per week
=A1/(((YEAR(NOW())-YEAR(B1))*12)+MONTH(NOW()))
will give you average per month