Open a new email, select Options in the Send Toolbar then check the Do not deliver before box. Select the first month's date from the calendar when you want the first mail to go out (and optionally change the time to whatever time that day that you want). Enter the text that you want and send the mail. It will remain in your Outbox until the specified time of delivery.
Just repeat this for each month, selecting another month each time. You can do as many months as you like, e.g. 36 months (3 years) or any number of months in advance. It will take but a few minutes one time and it will all be automatically handled thereafter.