Such a deep debate around french speaking ... It was not my purpose, so let's continue in english.
My OS is WinXP Pro SP3 (French)
My antivirus is Avast! 4.8 Home Edition (French because it's my mother computer, and her english speaking level is low)
My firewall is NVidia ActiveArmor v50.23 (French, with Nvidia 6150/NForce 430 chipset - ASUS A8N-VM CSM motherboard)
My email client is Thunderbird v2.0.0.23 (French)
The WinXP firewall is disabled, so I don't use another protection tool.
My firewall is not linked to this trouble : all my tests have been done with ActiveArmor disabled to.
Today, I have found a workaround to repair it, but it is boring to use :
1) I disable Avast! eMail Protection 'POP-Incoming mail scan' option
2) I restart my computer
3) I enable Avast! eMail Protection 'POP-Incoming mail scan' option
=> all is working
However, if I restart my computer with the 'POP-Incoming mail scan' option enabled, the problem is coming back.