Multiple Identities: Anonymity

[hede]

Hi list,

my users are using several identities for varying contacts. But some mailers add a "Sender" header with the main [uid]@[domain] email address. And using some mailers the local received-headers get stripped off and with others they remain intact.

While I think a header announcing "(SquirrelMail authenticated user [uid])" is okay and I even have an idea why this sometimes gets removed and sometimes not (postfix strips local headers with "127.0.0.1" in header_checks.submission but squirrelmail connects via the public IP; btw. I wonder why because it's configured to use "localhost".)
Thus this is ok, because everyone may know my internal user IDs...

But I don't know where the "Sender: [uid]@[domain]" header is coming from. It gets added with roundcube and android clients. Using Squirrelmail or claws-mail there is no such header. Is this client side or server side and how to switch it off?

Server is Kolab 3.2, single domain. I have installed squirrelmail locally for a faster alternate mail client (kolab runs off an arm embedded board), it's multiple times faster than roundcube.

regards
hede