Here's what I found, after a lot of methodical testing. First, the problem only occurs when the computer is booted up, logged in, logged off (without a reboot), and logged in the second time. That's right, it is on the
second login the Client Security build 6010 bug comes into play.
Since it was taking to long to test the problem with an e-mail client, I just started testing with portqry.exe to two different mail servers which accept port 25 connections. Under all proper conditions, portqry would get a "listening" response from either of the mail servers. After the second login (see above), however, portqry would no longer get a response from the mail servers, but instead was prevented from a connection. Again, the first login after boot, it has no problem, it is upon the second login the abnormal behavior is observed. This even happens when I disable the Symantec firewall, set the security setting to "manual", and reboot.
After testing all this to be able to reproduce the conditions upon the bug occurs, I uninstalled SCS build 6010, rebooted, installed build 6000, rebooted, and did my testing again. The portqry port 25 (and e-mail send) tests now work properly again, on the first, second, and third logins.