Kerio Connect is based on a lot of open-source technologies, and they do contribute back - but it is in itself a commercial product. For a small number of users, though, it's still a good value for those looking to DIY.
(disclaimer: Though I'm a user of it, I'm also a fairly large reseller by Kerio standards and my business gets a lot of our revenue from it)
The minus of Kerio is that it's commercial software and therefore not roll-your-own in nature. Limited tinkering is available. And to get updates after year 1, there's a subscription charge. The webmail is good but a little dated compared to some of the latest stuff out there.
The pluses, though, are these (in my non-biased opinion):
- Good antispam tech (blacklists, SpamAssassin, Bayes filtering). Not state-of-the-art, but traps most of it.
- Uses built-in Sophos engine and/or your own AV for filtering
- Easy to administer with web GUI, plus it's extensible with an API.
- Mail and config files are stored in plain text and can be accessed and edited by hand if needed.
- Supports native client for pretty much everything (Outlook, Mac apps, Sunbird and Thunderbird, etc.). Supports IMAP, CalDAV, and CardDAV.
- Integrates with AD or OD if needed
- Supports ActiveSync and if you have a Windows server it can support Blackberries (you have to run BES to do that, and BES is Windows-only)
- Easy to manage SSL, and it'll automatically use SSL for SMTP transfers if the target server supports it as well (so you get encrypted transmission)
- Runs on Mac, Windows, or Linux. Plus it comes as a pre-packaged VM for VMware or Parallels for appliance use. That's kind of handy.
- Scales well. It'll go from 5 to 1000 users pretty well on good-enough hardware. My largest client on it has an Xserve with an SSD boot drive and a RAID 1 mirror to support 1000 users.
They'll give you a 30-day trial if you want for free. And if you try it and like it, feel free to buy it from someone other than me - I don't get referral fees or anything for that but I'm not pimping it on my own behalf here.