utilizing external mail facilities from kolab server

[Realos]

hello,

I have successfully installed kolab server at our department but have
some questions about dealing with an existing mail server.

mail server: mail.dep.company.com
mail domain: dep.company.com

kolab server: kolab.dep.company.com
mail domain in kolab: kolab.dep.company.com

In kolab, I have set mail server (dep.company.com) as a relay host for all
outgoing emails. This works fine but has a big drawback with all users
ending up with two email addresses:

1) user at dep.company.com
and 2) user at kolab.dep.company.com

Since we already have a working mail server in our domain, can't we
configure kolab to utilize facilities of that mail server instead of
introducing it's own mail services?

I was thining of introducing the original mail server as a (slave?)
server if it does not require tweeking on that mail server. Is it easily
manageable?

Question2) We only need calender facilities of a groupware server, like
managing shared ressources, meetings planning etc. Can I use kolab
calender features without it's mailing facilites  and the other mail
server for all emails. How?

Any idea or hint to documentation is much appreciated.

Regards,

-- 
Realos