I would guess that the email address you are using in Thunderbird has a blank leading space. so it is being sent as " info" not "info"

I have carefully checked the sent messages and also my address book, but my e-mail addresses do NOT start with a space.

it could be that there is no info at that domain. delangstraat does not appear to be part of other rabo bank email addresses in the Netherlands.

The same message sent via the webmail service of KPN, or via hotmail.com goes well. The e-mail address then also responds with a correct confirmation. So the problem seems to be somewhere in my own Thunderbird. I have reinstalled the program, but this did not solve the problem.

If you sent the mail in TB through smtp.kpnmail.nl and it was rejected, but it goes through on KPN webmail or Hotmail, that suggests the rabobank has blacklisted smtp.kpnmail.nl, possibly because of it being used for spam or other illegal use. That wouldn't affect sending via webmail since it doesn't go through smtp. I suggest you try to contact the rabobank server administrators and see if in fact this is happening - or ask KPN for advice.

If you still think it's a TB issue, try sending with another email program, e.g. eM Client.

The problem is suddenly completely solved by itself. I have not changed or modyfied anything. Anyway, I am happy and thank everyone who has thought along.