Used Imail from IPswitch, neat and very simple to setup and maintain, but not free
Things you need to consider....
Even POP is not that cheap to get going.
You NEED multiple virus scanners, your "in house" will not catch all viruses. At least one, besides your in house AV, other wise you will get infected.
You will likely need to have anti spam add-ons, if you use free mail programs.
If you are doing this on the cheap with no backup mail server, make a deal with a mail service provider to accept your mail if your server goes down via MX record priority. If you go down, your mail is automatically sent to the provider... once back up again it downloads to your server
For a mail server use a raid 1 setup with decent sized drives, does not need to be a super powered server, POP is not demanding. You will need a static address and MX mail records. Don't forget to incorporate anti relaying, or the big boys will give you a hard time.
With 200 users you definitely need a mail server.
At one client, I spent more time dealing with POP mail then anything else..once they got a mail server, I spent about 1 hours per month on mail maintenance(70 users), nirvana.
Lastly, with 200 users, Exchange is a really nice system. Shame it cost so much to get going.
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Chernobyl disaster..a must see pictorial