I tried to send an email but without any success. Instead a dialog windows was opened and told me, that a message is copied to the folder Sent. But I see only a recurring progress bar.
New: Audacious (June 1, 2023), Platform 25.0 (Apr 07, 2023)
New Apr 14: macOS Portable Drive Support
450+ real apps (49GB), 1.1 billion downloads, Please donate.
I have used IMAP for the incoming mail server. Now I changed this settings to POP3 and the sending of an email is now sucessful.
This sounds like a permissions issue as Thunderbird was unable to either create your Sent folder on the server or store the sent email to it. You wound up sending me 5 copies of the email.
Sometimes, the impossible can become possible, if you're awesome!
In an article I found the reason for the above described issue. It is apparently the email provider, which does not support IMAP. Accordingly I created in Thunderbird another account for my email address by Google. And now my sent emails appear in the "Sent" folder.
EDIT: Here is an important supplement. Additionally it's imperative, that a setting in the email account will changed. Therefore you must rightclick on the left side on the appropriate email account. In the context menu you choose the entry "Settings...". Then choose the entry "Copies & Folders". Afterwards you must select on the right side the entry "Other:" under "Place a copy in:".