When implementing email security, you will have different clients with different budgets. Your client may be in the financial position where the cost of products, such as Microsoft Exchange, is irrelevant. In cases such as this, where the client has flexibility in the choice of product installed, your best recommendation to the client is the product best supported by the skill sets of current employees at the organization. But even though the client may be more comfortable with a server based on Microsoft Windows, their budget might require something a bit less expensive. In these cases, a solution based on Linux, using Sendmail or Qmail, might be the right answer.
Remember as you work with the client that you are considered the expert in this transaction. If you fail to provide the best information and guidance to the client possible, a future compromise could potentially be partially your responsibility.