I just downloaded a Plugin for the Claws E-mail program:
sudo apt-get install claws-mail-vcalendar-plugin
And according to the instructions, all you need to do is to open it
USAGE
Before using a plugin you must instruct Claws Mail to load it on
startup.
For this you must go “Configuration” menu on main window toolbar, open
“Plugins...” dialog, click on the “Load plugin...” button and select
the plugin file, named vcalendar.so, and press the “Open” button.
...But I cannot find vcalender.so in my home directory.
Any hints where I should look for it?