Solved The message has been successfully edited. NOT! Why?

rich,
I did what you wrote, but everything is the same ...
I go to post from the year 2010, push the button Action > Edit and than edit it and push the button Submit.

After that I get this messages:
Notice: Cannot save message: Invalid email address!
Message: The message has been successfully edited.

And post is the same like before the editing ...

And one more thing ...
If I'm editing the post, which is 1 month old, I can edit it without any problems.

One more thing - is it possible to edit posts somewhere else?
Like in backend, data base, or ... and not in forum directly?

You findt this in the /libraries/src/Mail/MailHelper.php line 128. I'm not sure but maybe it has to do with your problem.
I have no idea what it means:

We're a bit more conservative about what constitutes a "legal" address

One more thing - is it possible to edit posts somewhere else?
Like in backend, data base, or ... and not in forum directly?

In the backend it's not possible but in the database you can do it. You find the text passages in the table #__kunena_messages_text.
The easiest way to find the right message, if you enter at the search option of phpMyAdmin a short text passage of this certain post.
If you have more hits, choose the hit in #__kunena_messages_text and edit the message.

Thank you, rich!
I will do this through the phpMyAdmin - it works for me this way!