I admit, the frequency of that message is almost nil with OL2003 SP2 (POP3),
but I've still seen it there, and also in the OL2007 beta (not necessarily
the latest build).

So it was with great surprise that after all these years, I finally stumbled
upon a solution that looks like it might actually work. It's in Diane
Poremsky's newsletter, linked and quoted below. I did do some cursory
searching to see if it was already posted here, and if it was, I missed it.
It also doesn't appear to be in the KB, though I've seen searches come up
empty there before by mistake. Can't hurt posting again though, since it
seems like a FAQ.

Anyone currently experiencing the problem frequently, please report back on
success or failure.

Note that in my case, at least, I didn't have any of the three values
preexisting, so had to create them all (I doubt the one for Exchange even
applies to me, however).

http://www.slipstick.com/emo/2006/up060413.htm#popup

IS THE NETWORK POPUP GETTING YOU DOWN?

Most user find the popup messages about Exchange server messages, network
warnings & network connectivity is annoying after awhile, especially on busy
networks where there are frequent connectivity problems. While each user can
disable the alerts on their own, you can disable the alerts for all users
using a group policy or ADM file.

It's as simple as deploying a logon script that changes three registry
values found under
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\O utlook\Display
Types\Balloons

Change these values to 0:
Exchange
Netwarn
NetConn

By deploying these settings in an ADM file, with the default off rather than
on, users can change the setting, if they prefer to see the warnings.