designing a mail solution

I would assume you have no answers because this is a HUGE question that has 5million or more answers. :-)

SOme questions I would ask:

How many users? This will determine how many servers you need. It will also help you arive at a cost effective solution.

Price range for project? This will tell you how many servers you CAN install and what software options you CAN consider.

What functionality is expected? Calendar sharing and colaboration? JUST Email? Obviously Exchange & Groupwise are good choices if you need the whole ball of wax.

What is the expected ammount of message traffic? 1000 per day? 10000 per day? If there is a heavy messageing load, obviously you have to have a bigger or multiple servers available.

What is the current network infrastructure and Network Operating system? There might be a need to integrate into the current network. If everything is setup outside a firewall ina DMZ or seperate email authentication is not a problem, this is irrelevant.

What technical expertise is on staff to support the email? Is it going to be all outsourced or do they expect to handle basic administration?

Is web based email for traveling personel needed? Exchange2k has nice pretty web access.

Will there be multiple domains recieving email on the system? company.com, mycompany.com, theircompany.com...etc. Not all email servers handle this easily or efficiently.

There is a start for you. Hope this helps.