Dear Mr. Smith,
I would like to inform you that though I really appreciate the experience I have received in your company, it is time for me to move on, to take up new responsibilities. I have started to cooperate with a new company ready to promote my knowledge and experience in the sphere. I will be leaving you company in two weeks. The remaining time will be used for training any person you choose to replace me. Thank you for all our cooperation.
Best regards, Mr. Smayh