tcpdump script

[brad]

I have an ISP that does not have a record of their user's passwords and
we are converting them over to our system in a month.  I need to run a
script that will capture all pop3 passwords over the next month so I can
have a good record to enter into our system.  I started out using
tcpdump port 110 -w <file> and then use strings on the file.  I can see
all the USER lines and the PASS lines, but I don't know how to rework
the file to get USER/PASS in a readable and matched form.  I also need
to keep the file from storin all the other lines it captures so that my
file doesn't grow so large.  Any ideas?

Thanks in advance,

Brad