[imp] Default From Address for new users

[Binda Marco]

Hi all,

can anyone tell me how to set a default from address  to webmail users ?
My email server has Postfix smtp, Cyrus Imap, IMP webmail.
New users are created with webcyradm.

Users access the webmail with the Cyrus user and password

username  has the form:

user.domain   (it's the cyrus user)

(john.example.com)

If I don't set the from addr (empty by default) in the personal default identity , and send a mail, 
the receiver think my email address is:

user.domain at domain 
(john.example.com at example.com)

I need it to become:

user at domain
(john at example.com)

I tried to look inside the configuration files prefs.php and conf.php but I am a newbie
and can't find a documentation about Imp configuration

How cal I set the adrress "From:" in the  default identity of new users (when not already set) ?

Where can I find an in depth documentation on how to configure  IMP ?

Thanks in advance.
Marco.