Leaving a position can be difficult, particularly if you got on well with your customers. So no matter if you are retiring or going to another job, your customers will appreciate hearing from you personally, rather than having a strange voice answer your phone the next time they call. Customers feel hurt when they do business with someone who disappears without having the courtesy to let them know. Take the time to say goodbye properly, as it will stand you in good stead with your clients.
Address the readers by their individual names. It is worth the time and effort to print individual letters, even if the same letter is going to a number of people. "Dear All" is very unprofessional and sounds like a sporting announcement, rather than a serious farewell letter.
State the date when you are leaving. If you are retiring, specify that; if you are going to another position, indicate that it was a difficult decision to make. Introduce the person who is replacing you and provide her contact details, if applicable. Avoid making negative comments about the company you are leaving.
Express your gratitude for the opportunity to work with your customers. Thank them for their cooperation and support.
Recall a few memorable moments or situations. People enjoy hearing about anecdotes of your time with the company or your reflections on working with them personally.
Express your final thoughts and wish your customers all the best. Let them know that you are thinking of them in a congenial way, and that you hope they have good times ahead of them.
Provide your contact details if you want to stay in touch with your customers. This may work if you are retiring and will stay in contact with former clients. If you are going to a position within a competing business, it is better to just let them know where you will be without providing specifics.