In Skype For Business if you 'Reply with IM' to an email from Outlook, the Subject line will carry over to the Skype chat giving context to the message that you send. It will also provide a link to the rest of the email chain (As seen in the screenshot below)
Currently in Teams this does not happen. If you 'Reply with IM' to an email using Teams, it just opens a new chat window to the recipient without any context that links it to the email.

It would be great for this functionality to be replicated in Teams as it saves time explaining why you are messaging someone.