Email through sms is in separate threads When I receive an sms from an email address, the name of the thread is something like 0000000000000001.  If I respond, the person gets it, but then if the person responds to that, it comes in a new thread called 0000000000000002.  The next comes in 0000000000000003, etc.  I have tried adding the email address as a contact, but each new message still comes in a new thread.  What can I do to stop this?

I am using Silence (the messaging application) on a Nexus 6P on Android 6.0.1
There might be a problem with your message app. 
Unfortunately there is no possibility to get rid of this problem. 
You got a few options to choose: 



  Go to: Settings > Applications > Manage Applications > Select the Messaging
  App
  
  Choose 'Clear Data' and also 'Clear Cache'.



You will lose all data and text messages in your phone.


Try to install another SMS app there are plenty on Google Store. IMO the one of the bests is the Textra. Hope you enjoy it.
Maybe it is connected with an APN issue. 
This might help.