A mail server purchase is a major decision.  Not only are you making a substantial monetary investment you are trusting your online communication to a particular vendor.

For many, cost is a priority and there are a number of affordable solutions available.  Generally features scale with cost although some companies charge a premium for large numbers of users.

Most vendors offer a free trial which is an excellent way to determine if a product meets your specific needs prior to purchase.  Remember, to test all features during your trial, not just the features you plan on immediately using.  If you are making a substantial investment you don't want to have to switch products down the road if it can be avoided.

These days a big part of any mail server is its web interface.  Most systems either include one bundled for free or as an add on option.  For your users this allows easy access to email from any system with a web browser.

Before making your final decision get in touch with their support department.  Test their response time.  When you finally do purchase you will likely have some issues prior to purchase that need technical support.  Also check what support is included with purchase.  You may want to purchase an extended support plan if you support a large number of users or a company.