Open source mailserver

[kte at nexis.be]

I want to install an opensource mailserver on linux wit about 1600 users 
who send or receive about 10 messages a day for each user. He must have a 
web based admin + quota management + webclient + connecting from an 
outlook client (imap, pop3). I there an easy install/stable/configure open 
source mailserver that has these functions?
I' looking at openexchange, zimbra, more.groupware? But I don't have 
experience + they have more the just a mailserver + webclient

Thanks Koen
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