This feature is on the backlog. I will update you when we have more details to share.
Adding email as a left tab panel remains under review. We are working on adding several Teams and email integrations:
- Share to Teams: Share a copy of an email and any attachments into a chat or channel directly from Outlook
- Share to Outlook: Share a copy of a channel or chat conversation via email directly from Teams
- Actionable Teams Activity Emails: Teams missed activity emails display the latest replies and allow you to reply directly from Outlook
See this post for more information: