New email chain is started when I use Delay Send

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=== PLEASE READ BEFORE FILING ISSUE:
https://code.google.com/p/gmail-delay-send/wiki/GmailDelaySendDebugging_8
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What steps will reproduce the problem?
1. Reply to an email that was sent to me and schedule it to be sent for example 
at @14:30pm
2. Adding the label GmailDelaySend/ToSend
3. The email sends successfully (thanks!) but for the person receiving this 
email, it starts a new Gmail email chain, it is not appending the existing 
email chain, which is resulting in 2 physical email chains about the same 
thing. 

What is the expected output? What do you see instead?
Expected output is for the receiver of the Scheduled email, to receive an email 
within the same email chain, simply with the subject changed to "RE:...".

Instead, the receiver of the scheduled email receives a second, separate email 
chain, with the subject "RE:..." This should simply be appended to the original 
email, like any other standard Gmail email chain.

What version of the product are you using?
8

Please provide any additional information below.
Love love this program, thank you to everyone involved you've made such an 
improvement in my business, thank you!

Original issue reported on code.google.com by i...@phancybox.com on 26 Mar 2015 at 10:12

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Any feedback to this by any chance?

Thanks guys, this tool has improved my well being 1 thousand times over, thank 
you thank you thank you!! :)

Original comment by jcurn...@gmail.com on 30 Apr 2015 at 8:46