Very suspicious behavior in email client (Netscape)

I know nothing about hacking so I feel I need to share this with PLUG at
large.

I open a "new message" in my netscape client and if I type a string (in
the To: address field) such as 'PLUG' and press <Enter> I get a window
that pops open with over 90 email addresses with names where the string
'PLUG' is inside either the email address or inside the name. I
understand my client is searching for a similar string but where is this
list of names coming from? My cable-modem flashes activity and creates
this pop-up list when I press <Enter>, so I don't think someone is using
my machine to store email information.