The actual sending address can be shown in a column in the Threads Pane:

https://addons.thunderbird.net/en-US/thunderbird/addon/full-address-column/

Thank you, sfhowes, I know this fantastic addon, but "Sender (@)" cannot be used in junk filter definitions. These only use the columns TB provides. This was my initial approach: Either TB provides the original sender address and then we can use it in junk definitions or the addon is available to integrate "Sender (@)" in TB junk filter definitions. The genuine TB columns are not really helpful if I want to block email addresses or parts of them.

Best Dieter

I realize the add-on only provides a quick check of the sender's validity, but to filter junk automatically, you may have to identify certain text in other headers (custom headers can be added to filters). Even then, the effort may be futile, as unwanted mail constantly changes its form. Who is your mail provider? The major services apply fairly effective spam filtering, so much so that, in my case at least, I don't use TB's Junk Controls.

Thanks again, sfhowes It's really a pity that TB cannot provide the original email address as a header variable. This would help a lot. Should not be a big effort to include that variable. Maybe I should use OUTLOOK ;-) in the future?

Best Dieter

Check the message source (Ctrl+U). Is the actual sending address in the Received header? If it is, the Received header can be added to the filter criteria. The rule would be something like Received + contains + <all or part of original sending address>.