Thank you for your patience.
Do you have you email client set to check mail every x minutes?
No. I have thought it might have been something similar, too, but this is not the case.
What OS are you using?
Win XP SP2 unfortunately
What email client?
Thunderbird 1.0 RC
What other security software are you using?
Firewall and content filtering software
Using spam killers or something similar?
No, apart from the junk mail controls already included in my email client.
Try typing the IP into your browsers address bar and see if it resolves to a website. Or look at
http://www.dnsstuff.com/ to see who owns that IP
Already did, it says it's an address belonging to RIPE Network Coordination Centre. More probably ( I don't remember the exact address, and now it's not doing it anymore) it's a Dutch company of Internet services.