A Farewell letter is a polite gesture which communicates your thoughts and feelings as you are leaving a work-place, a neighborhood or friends. It is one way of showing that you cared about the people you were with.
Farewell letters are informal letters and must be written in a warm and friendly tone. They can be addressed to one individual or a group.
It is wrong to assume that farewell letters should be long and sentimental. Depending on the familiarity of your relationship, keep the letters light-hearted and positive.
It is good to explain in short, the reasons for your leaving. It is also a good time to recount the happy times you spent together or the experience you gained by serving together. Take the opportunity to thank the person/s for their contribution whether it was on a personal or professional level.
Farewell letters generally include new contact details. At this juncture, it is a good idea to express your wish to continue to keep in touch.