Email Reminders

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**
I am thinking of the following situation: A contact tells me to ask again after a period of x days if he has not managed to respond to my email request by then.

**Describe the solution you'd like**
After reading this message, I would like to be able to set an automatic reminder from Mail after a certain time x that I should become active again in this question.
So I know that I don't lose any important task and I still don't have to carry it around in my head all the time.

I'm just thinking, couldn't the feature also be connected with the Calendar app or existing/shared Online Accounts?