I didn't get any reaction from sh1tn's variation on your code, until I changed the port to 4025, as you described. I checked the various settings under Control Panel / Windows Firewall and didn't see anything pertinent.

Then I thought about my running Norton Antivirus with Email scanning active. I went into NAV and turned off email scanning (Options / Email). And gee, the code started working! Turned email scanning back on and your code stopped working.

Do you by any chance have Norton Antivirus or some similar antivirus software running on your Windows XP?