How to add a site mailbox to an already existing Team Site?

Question

How I can add a site mailbox to an already existing Team Site? When trying to activate the Site Mailbox feature, an error is is shown:

New Site Mailboxes can no longer be created in SharePoint Online. Consider using Office 365 Groups to connect a team site with an email conversation.

Answer 1

Update

Removal of Site Mailboxes in SharePoint Online

Status: Archive

Action Required by: March 1, 2017

Details: Beginning March 2017, new SharePoint Online organizations will no longer have access to the Site Mailbox feature, existing SharePoint Online organizations will no longer be able to create new Site Mailboxes; however, any Site Mailboxes provisioned prior to March 2017 will continue to function. In September 2017, we will have a process in place allowing you to transition your existing Site Mailbox site to an Office 365 Group. We will communicate again in June 2017, via Message Center, with more details. If you are using the Site Mailbox feature in SharePoint Online, you should consider transitioning to Office 365 Groups, to support email-centric collaborative scenarios.

This is quote from Office 365 Planned Service Changes for 2017 – Admin Action Required. So as stated, the Site Mailbox feature can no longer be activated with SharePoint Online.

The alternative as provided by Microsoft would be to move to Office 365 Groups. O365 Groups support email-centric collaborative scenarios, which should fulfill e.g. SharePoint's Site Mailbox functionality. Learn about Office 365 groups.

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You can active Site Mailbox by going to Site settings > Site Features > Activate Site Mailbox.