Why can't I reply to a co-worker's boxed email?
What you are seeing is Streak's "Unified Thread View". This is a wonderful example to demonstrate how Streak interacts with Gmail!
When a user adds an email to a Box, Streak will 'bookmark' the email's identifier future reference. The next time a users access that boxed email, Streak will search the users account to see if they have ever been included on a copy of the email (as the sender, as a direct recipient, or CC'd/BCC'd). If Streak is able to find the email in your inbox, we will direct you to that email thread in your inbox. If Streak cannot find this email in your account, we will provide read-only access to the email, indicated with a the blue icon.
Streak can provide read access to these emails, but cannot provide a user write access to another user's email unless they were included on the email thread as well.
Google Groups are a great solution to team email collaboration. Google Groups sends a copy of the same email to the entire team. Because all team members have access to this email in their inbox, they will always have the ability to reply to the original email that was sent.