@alexisgarcia, I agree with your suggestions as well, and most of all, gigabit connections help immensely, especially if you are using Exchange.
I would suggest stopping the use of PST files if you are only supporting Exchange users (no POP3 or IMAP from other email sources).
We are using Symantec Enterprise Vault, but I think there may be better solutions available. We have a lot of messages with large attachments, and using the mail archiving solution has helped me maintain a manageable exchange database. I think this also helps Outlook run much faster too.
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