Sieve over unsecure networks

[Michael Pasdziernik]

Hello,
My KDE Kontact connects to a kolab server over unsecure networks
and the groupware wizard has configured sieve filtering.
There are several questions concerning sieve security:

Are sieve connections encrypted by TLS/SSL? 
Is the same authentication mechanism used as for the imap connection?

If it is not secure: 
Is sieve needed for the main groupware functionality or
just for filters that the user can define?
Does Toltec use sieve also?

Thank you and best regards
Michael