Design a software system to resend emails which are forwarded to a special formatted email address, for example to email@example.com, at the given point in time.
Such a service exists at http://followup.cc. Design an architecture for such a service. You can use followup.cc for free. So maybe it helps to first explore what the service has to offer.
As a user I can forward emails to a special formatted email address at the followup.cc domain. The email address defines the point in time when the email is resent to me to remind me to follow up.
Examples for email addresses:
In 7 days from now on.
In 12 hours from now on.
At next august the 15. at 9 A.M.
In one week, three days and five hours from now on.
When forwarding an email to firstname.lastname@example.org the email will be resent to me in 7 days. The service will resend the mail at roughly the same time as I forwarded it to the service. I may use the service as a reminder to follow up on the mail.