Open relay?

[Maxim Sorokin]

Dear Sirs!
Can you help me find a problem with mail server:
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Kolab2 Groupware Server Version
2.0.1rc1

Kolab2 Groupware Server Component Versions
perl-kolab-5.8.7-2.0_20050727
kolabd-1.9.4-20050801
kolab-resource-handlers-0.3.9-20050727
kolab-webadmin-0.4.0-20050728

Kolab2 Patched OpenPKG Package Versions
amavisd-2.3.1-2.4.0
apache-1.3.31-2.2.3_kolab2
imapd-2.2.12-2.4.0_kolab
postfix-2.2.3-2.4.1_kolab

OpenPKG Version
openpkg-2.4.2-2.4.2
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Several providers wrote us that a lot of spam started to go through our
server about 1 month ago.
How can I check is it security bug or settings? May be some users got a
virus or trojan - how to check their IP? 
We checked that the server is NOT open relay (directly, but I know there are
some ways to change headers and use non-open relay).
Check Postfix log?

And second question is should I upgrade the system (Kolab) from 2.0.1 to
current stable release 2.0.3? And how to do it more safty for settings?
The problem I've never use Kolab (last Admin set it up), but I worket with
Postfix.

Thank you

Best regards,
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Maxim Sorokin