The issue is probably the reply to address. They do some strange things at outlook.com.

Thank you. But how can I solve it and get it working properly?

does changing the reply to work? I am only guessing.

Solve it by sending GMX mail using the GMX sMTP server instead of Outlooks.

Thanks. I have already set up Thunderbird to receive email from gmx and to send replies from gmx, and that is working ok. The problem is when I receive from outlook. If I reply to these I want to set the "reply to" to gmx, and I have tried different options as follows. A) outgoing server set to outlook, reply to set to my gmx email. In this case message is sent but error message comes back as described in my original post B) outgoing set to gmx and reply to set to my gmx email. In this case, the message is not even sent and there is an immediate error message "An error occurred while sending mail. The mail server responded: Requested action not taken: mailbox unavailable Sender address is not allowed.. Please verify that your email address is correct in your account settings and try again."

So setting the outgoing to gmx appears to make things worse.

For info, all this started when gmx asked me to change my password which I did. Is it possible that Thunderbird has the wrong password(s), either for gmx or for outlook or for both? And if so, how do I change them?

http://thunderbirdtweaks.blogspot.com.au/2011/07/pssswords.html

Thanks Matt. Used the website address you gave to check passwords, and these were in fact ok.

So I went to Outlook and found an error message. It was using the old pasword for gmx and it needed updating it because I am forwarding received emails from gmx to outlook. Changed gmx password in outlook and all is now ok.

Thanks for the help