Open Source Vs "Enterprise" mail servers

"I've decided to go with Exchange 07 as my email server but I'd like to discuss some advantages/disadvantages of open source servers ... Any ideas on what reasons I could give for going with Exchange over Open Source? "

You have the process completely backwards. You gather the information, examine the pros and cons of all options, and then make a decision. You don't make a decision and then go looking for data to support it.

If you don't know the advantages and disadvantages of the options, how did you decide to go with Exchange? Ah, I see, you let the vendor influence your decision. Never a good idea; he's working in his company's best interest (that's what he gets paid for), not your's.

For an installation with only 350 users already on a Windows network, Im thinking about going with Exchange since it integrates so nicely with Sharepoint, Outlook (bundled with Office 07), and provides all the calendaring and collaboration features that are really needed in the hospital enviroment.

(At present the office communicate via paper memo)

If the installation was 5000+ users I'd probably be looking for something else, and if it was really mission critical they wouldn't be using Windows servers in the first place.

Im a little reluctant to go with an open source option because project can die and also support issues. I want to mention Note/Domino but I feel it will have too many "not needed" features for hospital system

1) Is the traffic of both an AD server and a mail server on the same box going to cause excessive disk thrashing on the server? Dual raid arrays on the server can help minimize this, but you will need a box with 6 drives.
2) What is the long term expected use of email, and what is its importance? Many people say "I can live without if I have too," but then throw a huge tizzy the second you reboot the server.
3) Are licensing costs an issue?
4) Are their any regulations concerning security rating standards for the servers? (Applies more to govt then hospitals, but ya never know.)

Advantages to exchange, simple AD integration. Integration with Sharepoint etc. Pre-installed from Dell. relatively easy to manage.

Advantages to Qmail, Sendmail etc. More robust, better management of mail queus (native, not including expensive 3rd party options). rated for secure government use on a unix platform. Less down time from patches and updates.

Dont forget, there are commercial *nix mail servers out there with support. Exchange or a home built serevr are not the only options.