Edit server settings in Thunderbird

Please rightclick your mail account (see screenshot: 1) and click on "Einstellungen"/"preferences", the settings windows opens.

In this window please click on "server settings" and rename the entry to:
imap.leibniz-zmt.de (see screenshot: 2).

Also rename the outgoing server in the settings window, to do so please click on outgoing server,
(make sure your mail account is selected), and click "edit" (see screenshot: 3).

In the popup please edit the field "Server:" to:
smtp.leibniz-zmt.de (see screenshot: 4).