Notes:

This particular client was extremely happy with an earlier Open Source substitution
that I had put on-site that year. As a result they seriously considered replacing
their current external mail router with an all Open Source solution (no software cost,
no licensing fees). Sendmail was their Open Source MTA of choice - they had some limited
experience with Sendmail. I also talked them into using a mail policy application as well
as an Open Source Anti-virus application on the server.
The cost of the cluster implementation was only +$2k more than their annual expenditure
on their current single server and gave them a fully redundant server pair with greater
functionality and no on-going licensing fees.