I am currently looking for a good open-source groupware solution that supports push, mapi and a synced calendar. I have been looking at kerio, openxchange, openchange, zimbra and zarafa so far.

After installing Zimbra I was very impressed, but unfortunately the community edition did not contain these features. After calling the sales-department I was disappointed to hear their minimum number of accounts is 15 and I am still undecided after the 60 days of testing (trial).

I would like to have some personal recommendations regarding the choice of Linux Exchange alternative.

with Scalix Community Edition you get 10 premium users and unlimited standard users. With these 10 premium users you can use Outlook MAPI & Evolution Connector, however for ActiveSync you have to go with Small, Enterprise or Hosting editions of Scalix.

I have requested a quote from Scalix for 15 users to see what the price per-user will be when I reach the limit of 10 users. Unfortunately I expect high prices that near the price for Microsoft Exchange.

from what i remember it's cheaper than exchange, higher than zimbra, however their pricing depends on region/market, so you might get better deal.

also note that if you start with Community edition and need more premium users at some point, you cant get additional premiums per user as far as i know, but you need to order starter packs, which comes with 5 or 10 users at once.

I would suggest Kerio.. I have been using it for awhile now and could not be more pleased by it's performance, capabilities and affordability.