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By configuring your Microsoft Office SharePoint Server 2007 site to receive e-mail messages, you can enable team members to use e-mail to send information, messages, and files directly to different areas of the site. For example, users can send announcements directly to the home page or send documents as e-mail attachments directly to document libraries. Integrating e-mail can make it easier for your team to communicate — and give you the benefit of tracking discussions and key decisions on an Office SharePoint Server 2007 site. Watch the demo to learn more.
Don't forget to carefully configure yôur SMTP Virtual Server Settings. Specially the Security Settings and Relaying. If possible make strict restrictions for the number of connections and the number of recipients !
How to do it (text version):
Read also this pdf
Plan Incoming emails (by TechNet)