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If you check multiple email accounts from one gmail address, you will probably have had this problem at some point.

So you have got two addresses, your main account test712@gmail.com and dontspamme@gmail.com.
When an email is sent to dontspamme@gmail.com you want to respond with the same address not your default (main acocunt) test712@gmail.com.

To do this got to your account settings at the top right of the page, then select accounts.  In the section send mail as section look for When I receive a message sent to one of my addresses:

You now have two options
Reply from the same address to which the message was sent
Always reply from my default address (currently test712@gmail.com)
Select the correct one for you and your gmail is now the way it should be.

Please remember that you can, if you want select the email account to send from when composing a message!