You should also configure the "reply-to" addresses on all the mailboxes within Exchange to point to the SMTP addresses...
that correspond to your users' POP accounts. Once this is set up, Exchange will be able to send outbound Internet mail even without a static IP (it'll just need a reliable DNS server to point to -- resumably the one configured by your ISP) and anyone replying to these messages will send those to your users' POP accounts. Once all that's configured then you can absolutely use Exchange Server 2003 for messaging and collaboration – specifically storage of mail and centralized contacts as you suggest -- within your company. Don't forget to get some good backup software that is Exchange-aware to make regular backups of your server.
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