Microsoft Exchange alternative

Looking for a freelancer to help me find and implement a self-hosted alternative to Microsoft Exchange. My primary goal is to save costs, so I'm looking for someone who can recommend affordable solutions that will still meet my needs. The alternative solution must be able to run on Linux, as that is the operating system I am currently using. Ideal skills and experience for the job include:

- Knowledge of affordable and reliable self-hosted email solutions
- Experience with Linux operating systems and server management
- Familiarity with Microsoft Exchange and its features, in order to identify suitable alternatives
- Ability to provide clear and concise recommendations, as well as implement the chosen solution with minimal disruption to my existing email infrastructure.
- ability to sync with activedirectory
- no cost of software