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Machine sizing and network design are part science, part black art. The number of employees in the various locations is a major factor in designing the solution, but other factors need to be considered as well.
For example, If your IT and corporate strategy involves centralization or server consolidation, you will want to host all mail users on a single mail server. This has many advantages, including less cost and complexity -- specifically, less hardware and configuration complexity to purchase, install and maintain your e-mail system. Such a strategy, however, may require you to increase the network bandwidth between remote and HQ offices to ensure that end user performance is adequate. This is greatly dependent on e-mail client choice and means of connectivity. For example, POP/IMAP based clients will not be as bandwidth sensitive as MAPI-based clients, like Microsoft Outlook. Ultimately, the centralized configuration will be the lowest in cost.
Alternatively, if each of the three offices maintains a high degree of autonomy and you have IT staff at each location, you could host a mail server at each office. Because the decentralized design requires additional hardware and IT effort to manage, this is typically a more expensive configuration to acquire and more complex to maintain. It could offer higher perceived performance for each group of local users, if you don't have the ability to deploy the network bandwidth required to support a centralized server.
Some of the other factors that go into your design include the amount and nature of the traffic between the various clients and servers, the amount and nature of traffic between the three offices, the cost for network bandwidth, growth plans and IT skills present in the remote offices.
You'll want to confirm machine sizing with your mail server software vendor. Many, but not all, Linux-based mail servers can support 500 "office workers" on a single server-class computer configured with sufficient memory and disk storage. I'd recommend evaluating Sendmail, Cyrus IMAP server for basic e-mail, and Groupwise, Notes/Domino, Bynari, Scalix and OpenExchange for more advanced e-mail and collaboration capabilities. It will be important to understand your end-user needs in terms of functionality as you perform your evaluation.
With e-mail clients, there are a number of choices for Linux, Windows and other desktop systems. A good e-mail client is one that is compatible with mail server software, supportable by IT and familiar to end users so that additional training isn't required. Microsoft Outlook is frequently used on Windows desktops. Novell Evolution and Mozilla Thunderbird are popular choices for Linux desktops. Several Web e-mail clients now rival the features and performance of full desktop e-mail applications, and are worth consideration. E-mail clients and Web browsers can usually run on minimally configured desktop computers.
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