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There are apps on the App Store that can do that. The one I'm using is not available in all Countries, but there are others. I'm sorry I cannot say the name of the app I'm using, since the forum rules forbid it. Just search for it.
 
Check www.tickletrain.com - it allows you to send emails at a different time, but really helps with managing your follow-ups if that's what you are trying to do...

I am not sure TickleTrain is what they need. TickleTrain is great if you want to follow-up on emails and have them sent automatically for you at a later time if you do not get a response. But if you simply want to send a response to an incoming email at a particle time I have not see such a plugin but sounds like a great idea!
 
I appreciate the comments on SendLater. I have been looking for something like this for over year. Works great!
 
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