I suspect that this is part of the mail provider pprune uses.
Am I right in thinking that it's 'powered by everyone.net'?
I have another mail account using this service and suffer the same problems as Binos but only on my home machine (WinXP), work (WinXPPro + Linux firewall) and my old 'number 2' (Win98 - believe it or not) are free from it.
When I get back I'll have a go at investigation a bit more.
VnV