11.10.2.How Mail app works in MS Outlook 2013+ and OWA 2013+
After you have installed Mail app it is recommended to restart your Outlook.
Open your Outlook desktop, and select a message including an attachment that you would like to archive manually (1). In the reading pane, click on contentACCESS Mail app (2)… Archive email (3)… Archive (4) buttons.
In the reading pane, click on contentACCESS Mail app again. To view the original email, click on Show original email option. To download the archived attachment, click on it in the message and it will be downloaded automatically.
Note: In Mail app the “split attachments” function is active by default. That means that the body of the email and the attachment are stored separately. It is good to know: if the email message containing an attachments has been previously archived by a job, where attachments were not split, then the mail app in MS Outlook will show 0 attachments.