I would like to obtain data on the growth of email traffic (in 
Gigabytes) over time for the last several years.  There are a couple 
of different ways to answer this question: 
1. Total volume of email (in Gigabytes) transmitted or received by email users 
over the last several years. 
2. Average amount of email (in Gigabytes) seen by a typical email user 
over the last several years. 

Clarification 1: I don't care if the data is from the US only, or worldwide. 
Clarification 2: The data could be for corporate (intranet or internet) 
or home-use email, or for any combination of these three. 
Clarification 3: I would like to obtain this data for several years, 
at preferrably several good data points, including the current year 
2003.  For example, for the years 1991 1993 1995 1997 1999 2001 and 
2003, or something equivalent that can be plotted on a graph and 
projected into the future.  If you can only provide data for two or three 
years then that would probably be ok. 
Clarification 4: I already have some information from a study done at 
UC Berkeley a few years ago.  Please don't bother to refer me to that 
data as I have already seen it. 
Clarification 5: I am not interested in the number of emails or the 
number of email users.  I am only interested in the volume of email in 
Gigabytes. 
Clarification 6: You don't have to distinguish between inbound email 
and outbound email. 
Clarification 7: I am not interested in spam mail.  If the data 
includes spam as part of a larger total, that is fine, but I am not 
interested in knowing the volume of spam mail only.