Follow up with Outlook. Set a follow-up flag before you send an email by using the Message tab, which appears in place of the Home tab when composing mail. Look in the Tags group and choose the Follow Up drop-down button. Select one of the preset follow-up times or a Custom date. You can even set a reminder.
This is especially helpful for billing. For example, if you send bills by email and the payment commitment is 30 days, set the follow-up for 31-35 days.