That's a "real-life" situation that we run into many times when we've gone out and worked with companies. We might be called in by a company that has both a Microsoft Exchange and a Notes/Domino email system and they want to tie an archive together, so that the content is searchable across the whole enterprise. This usually happens when one company (with Exchange for example) purchases another company with another email system. As they integrate the company, they finally move to combine email systems or create an archive and the difference in email servers presents a problem.
Just a few years ago, the answer to this dilemma would have been two archiving systems. Since 2000, the vendors have gotten pretty good at combining disparate email into a central archive. Not all vendors support this (it's a good question to ask an archive vendor), but most of them do have that ability today. It's very painful to put two email archiving systems in place; it's also very difficult to go backwards. Discuss these concerns with your vendor ahead of time and you'll save a lot of time and money.
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