Remember that email (e.g. Microsoft Outlook) is a communications system and should not be used for storing files.
Save any attachments you need to keep to OneDrive in O365 or add them to your team SharePoint site then delete the email. If you are sending attachments, if possible send a link to the uploaded document on O365. When saving a document, ensure you follow the naming guidance and file the document under the right heading of the BCS on your SharePoint site.