It depends on whether you want to try to persuade the person to stay with your company or not! If one of your key workers resigns you can take the letter from him/her but arrange a meeting to go through the reasons before you write to them to accept their resignation. Before you meet though, think about whether it could be related to salary and calculate what salary you may be able to stretch to if that is the sole reason for the resignation. If it’s a non-key worker take the letter and confirm that you will acknowledge the resignation in writing and will provide details of the arrangements made for the end of the employment.