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INTRODUCTION

Microsoft works with public X.509 certification authorities (CAs) to make sure that Microsoft Exchange Server customers can obtain the certificates that they require in order to use Microsoft Exchange Server 2007 for the Domain Security feature to enable secure client access from the Internet.
Microsoft also works with CAs in order to make sure that Microsoft Office Communications Server 2007 customers can obtain certificates that they require for Communications Server 2007 servers.

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Good post from Aaron on the topic that is worth a look…