How do you write a letter to confirm that you have known someone for a long time?

To write a letter confirming that you have known someone for a long time, start with a formal greeting and clearly state your relationship with the individual. Include details about how long you have known them and the context of your relationship, such as personal or professional interactions. Conclude by affirming your positive regard for the individual and offering to provide further information if needed. Remember to sign off with your name and contact information.