I've found that viewing messages as threads is especially useful in a business setting. Among its other improvements, the latest version of the Mac operating system, Mac OS X Snow Leopard, is more enterprise-accommodating with more seamless and easy-to-setup support for Microsoft Exchange. With it, you can view your Exchange emails, access Active Directory features, and sync your calendar to iCal. The only catch is that it requires Exchange 2007 or higher.
To add an Exchange account, make your way to the "accounts" pane in Mail.app's preferences. There, you'll need to click on the "+" button on the bottom left to add a new account. In the first step, fill out your name and the email address of the Exchange account you want to add. In the second step, for account type, choose "Exchange 2007" and fill out the accompanying incoming mail server, login and password. Hint: the mail server sometimes is the same one you use to access your Exchange webmail, sans the "/exchange." After this step, Exchange's Autodiscover feature should automatically fill in the rest of the fields.
Take account offline
While Exchange synchronization provides a connection to your office, sometimes it's nice be disconnected. To take any mail account offline (this includes your Exchange and all other accounts), right-click on the inbox or any folder within it, and choose the "take account offline" feature.
If you have many accounts and folders, sorting through things may prove burdensome. However, smaller mailbox icons may make viewing easier. To view your mailboxes as small icons, right click on any mailbox or folder and choose "Use Small Mailbox Icons."
One feature of Mail.app on Mac OS X that I wish its iOS counterpart had is account-specific signatures. In other words, you can have a different signature depending on which account the message originates from. To create a signature, click on "signatures" from Mail.app's preferences. Then, choose the account that you want to create a signature for and click on the "+" button on the bottom. Fill out your signature on the right hand pane, and you're all set.