As part of a security measure Google will not allow modification of email content by applications. Therefore, to insert OnINBOX's A C T banners into emails, we duplicate the email, inserting the banner as the copy is created. Once we verify that there was no issue with creating the new copy, the original is moved to the trash.
A drawback of this is that Google will notify you that there are deleted messages in the conversation thread whenever you view a message. To stop this from happening, you can use the Permanently delete feature, which will permanently delete the original message instead of moving it to the trash.
This setting can be found in your OnINBOX personal panel under Settings (Shown below). It is disabled by default.