[imp] How to specify default preference settings for all users?

[Nikolaos Milas]

Hello,

Where can I find a list of available settings so we can know how to 
specify them in a prefs.local.php file.

For example, I want to specify as default settings for all users (as I 
see them stored in horde_prefs db table for a particular user):

Scope         Name                  Value
-------------------------------------------------
horde        sending_charset        UTF-8
horde        timezone               Europe/Athens
imp          default_msg_charset    UTF-8
imp          max_msgs               200
imp          use_smime              1
imp          initial_page           INBOX

How can I define them properly in prefs.local.php?

Where in the documentation can I find a list of all available settings 
(and their possible values) in the proper format so as to be added in 
prefs.local.php to become default values for all users?

Thanks in advance,
Nick