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I'd pitch it like this:
"Email is a business-critical function. There is known difficulty in achieving a reliable solution with Microsoft software even for an organization with essentially unlimited resources to put onto the job (e.g. Hotmail, with all Microsoft's resources behind it.) It would be therefore be extremely poor judgement to implement a project of this scope with no alternative project strategy, given that failure might mean catastrophic loss to the business. Therefore, if they want to attempt a solution with Microsoft, the *only* prudent strategy would be to concurrently implement two solutions, one based on Microsoft and one based on software successfully in use for large numbers of users at Yahoo and other ISPs, namely Postfix.
"Note that this number of users is large in the corporate sense, and large for Exchange installations, but is at most a moderate-sized userbase for most ISPs. Most ISPs consider anything less than 100K users to be small, hence software designed for ISPs will easily handle a customer base of only 20,000 and so is a reasonable alternative to evaluate.
"With the two-pronged approach, if either strategy fails at some critical point, it can simply be dropped and the other one can be carried to completion. Because of Postfix's open source licensing and ease of setup, using it as a 'backup' strategy is low cost both in terms of direct costs and labor costs."
I would also define some hard benchmark numbers, based on actual usage, which you can reasonably demand that both MTAs be held to, and which you can actually test on dummy users during the course of this conversion project. (In fact, I think a reasonable project proposal would devote equal resources to advance testing of the system as to the actual conversion.) At that point if the MS MTA fails the tests, you have a reasonable argument for demoting it to the backup strategy and later dumping it if Postfix continues to out-perform as you expect.
Some random additional points to cite: high cost of a successful outside intrusion/attack to the email servers (Postfix is designed for security, quote from the overview); the fact that people on this mailing list are routinely sending out lists of several million users with Postfix.
© by Ralf Hildebrandt
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This file was last modified 24. Sep 2007 by root