A goodbye letter to a co-worker is written to advise him you are leaving the organization, say goodbye and let him know you have enjoyed working with him. While you may express regret in leaving, it should remain positive, steering clear of any negative memories or experiences.
The letter should mention any particularly memorable moments or periods of your time together. Thank him for helping you achieve goals, contributing to your growth and providing support. Let him know you cherish the memories you have of working together, and offer him best wishes for success and happiness in the future. If you wish to remain in touch with the co-worker, you may include contact information in the form of an email address or social media account.
The letter should not express happiness about leaving or finding a better position, and it should avoid any references to negative experiences. Some industries are small enough that you may cross paths with some previous co-workers. Since you may see some at professional group meetings and networking events or even end up asking one of them for a job some day, you should depart on a positive note and leave your present co-workers with the best possible impression of you.