When an e-mail message is sent to an Office 365 Group, it is automatically sent to all the Group’s members and recorded under the Conversations tab of the Group. Sometimes, you may receive an e-mail which was perhaps only sent to a subset of the members of the Group, but you wish to save the e-mail into the Group’s mailbox. The normal way to do this would simply be to forward the message to the e-mail address for the Group, but what would happen is that everyone who received the email before, would receive it again. It would be ideal if the e-mail could just be placed into the Group's mailbox, for example for documentation purposes or just to make the information available to all the group’s members for historical purposes - without having to send the e-mail to all the group members again. The following explains how this can be done:
- Go to portal.office.com and sign-in.
- On the list of Apps, click the Outlook icon.
- Select an e-mail message or conversation from your mailbox that you want to move to the Group mailbox.
- Drag the message to the desired Group.
NOTE: You can currently only drag and drop one conversation or 100 individual messages at a time.
If the original sender or recipients on the e-mail aren't Group members, they will not be added to the group through the drag-and-drop process. They won't be notified that the message has been moved to the group either. Those who are already following the group in their inbox will not receive a copy in their inbox.
Adapted from https://support.office.com/en-us/article/Drag-messages-from-your-personal-mailbox-to-the-group-mailbox-24e809db-70e1-45b7-8d54-efac7951dc95 )