I could really create a monster with this one. FollowUpThen is free service to follow up on emails sent with an automated email reminder. If you send an email to Roger and ask him to follow up with you in 5 days copy firstname.lastname@example.org. This way if Roger fails to respond within 5days he will receive a nicely formatted reminder email with the original content.
There is nothing to install, no special configuration simply address your email as you normally would cc or bcc the time duration and wait for the reply to come back.