User licenses aren't the only software cost. Antivirus software is absolutely essential for security if you choose to operate your own email servers. Not only will this software have to be deployed on the server, it will also have to be installed on every computer that uses that server. Each of these will require a license as well, and that means more per-user fees.
More major costs can be incurred when a large-scale software or hardware upgrade becomes necessary. This is often the reason businesses first start to investigate the outsourcing option. Technology doesn't stand still, and what was sufficient a year and a half ago may not only prove to be outdated in short order, but may be rendered completely incompatible with current standards.
Changing hardware or software also often means that employee training and troubleshooting in the first few months will continue to put a financial drain on your business. The system changeover itself will usually cost between $100 and $300 per user.
