Instead of a regular stamp, the sender used a Pitney Bowes postage
meter. The meter said ?mailed from zip code 60xxx?, and there is an
account number ?0004388xxx? (I?m not showing the last 3 digits for
privacy reasons).
Is there a way to find out the name of the company that has been
assigned this account number?

We have a Neopost machine and the number it prints out on the indicia
is the meter serial number, not an account number.  In the past the
local post office validated the meter and had a record of who had
which meter.  Now Neopost has that record and I guess it is the same
for Pitney Bowes.  Can you find out who has the meter?  I can't think
of a legal way!!

http://www.pitneybowes.co.uk/