You can now attach emails to other emails in Gmail without downloading them first. This is useful for situations where attaching emails makes more sense than forwarding separate emails, like wanting to forward multiple messages related to a single topic. With this new functionality, you can do exactly that. Sending emails as attachments allows you to write a summary email message to your recipients, and attach the set of supporting emails that recipients can directly open in their mail client.
To insert emails in a new thread, drag and drop them into the draft window.
Another way to insert emails in a new thread is to select the emails and then from the overflow (three-dot) menu select “Forward as attachment”.
To reply to an existing thread, you can pop-out the compose window and then drag and drop emails to it.