This e-mail is allways marked as possible scam.

What does this mean exactly?

You're right, Christ1, I should have written "These e-mails are allways marked as possibele scam", referring to the e-mails sent by the helpdesk I mentioned. That is to say, I receive e-mails from one particular sender that I trust. Above these e-mails I find a red line with the text: "Dit bericht kan een scam zijn", which is Dutch, meaning: "This message could be a scam". I don't like that, and therefore would appreciate it when I could exclede this sender from the scam-survey.

The scam detector is unfortunately both stubborn and stupid. You can't teach it, you can't set up exclusions, and it doesn't recognize your Address Book as a whitelist (as does, allegedly, the Junk detection system.)

It looks like I have to live with the shortcomings of Thunderbird as long as I keep using it. I don't want to complain since it's just a minor issue.

Well, you could disable the scam detection feature. See https://support.mozilla.org/en-US/kb/thunderbirds-scam-detection

I knew that from the beginning, but thanks anyway. It is a solutiion, much heavier than I hoped for, but I think I may just do that. I think I'm well aware of the dangers of scam and I hope I'm right. Thank you all for your comment.