Why are you not using the gmail smtp server?

Because I am trying to help my sister who has a kpn. account. She uses Mozilla thunderbird as mailclient (don't know why, probably because someone else installed it). I myself use webmail.

But the email account is run by googlemail and they provide an smtp server. The ISP is quite understandably refusing to "relay" messages with a "From:" address belonging to a domain that they don't control.

I suggest you set this account up to use smtp.gmail.com on 465, with SSL/TLS, OAuth2 and the user's login and password that they use for incoming mail.

Webmail? Ugh! ;-)

I think you don't quite understand my problem. My sister has an @planet.nl emailaddress. On her laptop she has Mozilla Thunderbird as emailclient. On her laptop, with her accountsettings in her Thunderbird, I tried to send an email to myself at my own emailaddress (to be read on my own laptop). This is the same situation as if my sister would send an email to me. This is exactly the problem: that doesn't work. PS: when I set up an account on my sisters laptop as another user, so with another useraccount in the mozillaclient with my own emailaddress (so a new account, separate from that of my sister), everything works fine, I can send emails to other people, myself and my sister. So: something is wrong in my sisters outgoing server settings, but I can't figure out what and I can't find what this specific error code means. I hope I managed to explain my problem now. Thank you for your time

The message says to me that you have asked a server at kpnmail.nl to send a message on behalf of p.a.weterings@gmail.com. No mention here of planet.nl.

The same happens if I send a mail to pwet@live.nl, so it isn't the recipient that is at fault!

You say it's the same (or a similar?) error message? Does it complain that you're trying to send a message from a planet.nl address through a kpnmail.nl server?

Who is your sister's ISP? All you've given us so far is about email providers.

The recipient (To:) isn't the problem here, I think. It's the sender (From:).

Sorry, but if you only ever use webmail, you won't be familiar with the specific details of how email clients and email servers work together. What you have here is a classic problem that email client users can get if they have a substandard email provider and try to use their email client when away from home.

A good email provider will offer an smtp server that uses authentication and doesn't care where you send from. But I am guessing because you haven't shown us the error you saw when you sent with the planet.nl address.

I discovered what caused the problem. As I didn't get much further with your "answers" I contacted the emailprovider. It seem that @planet.nl addresses are obsolete, and only maintained to please customers who don't want to change their emailaddress. However, they can only use this address from their homes! (I took my sisters laptop with me to my house to update to W10). So I adjusted the settings in the webmailapplication and everyting works fine. Thanks very much for all your trouble.