Where could it be? Between clutter, trash, inboxes, and folders it is possible to have mail misplaced, but if you have looked in all location and even used the search feature you could be missing something. Microsoft Outlook is constantly working behind the scenes to ensure your mailbox safety. Your missing mail may have been Quarantined!

Here are the steps to save that missing mail:

From the Office 365 Security & Compliance Center:

Quarantined email can be viewed in the Office 365 Security & Compliance Center.

1. Go to https://protection.office.com/#/quarantine. 
2. Login (if necessary)
3. The Quarantine window should already be displayed (a list of quarantined emails will be displayed if they are present). For reference the quarantine listing is found under “Threat management”  and then under “Review”
4. Click any message to select it, then choose from the options given:
   • Release message
   • View message header
   • Preview message
   • Download message
5. Remove from quarantine
6. Release message: In the popout pane that appears, choose whether to Report messages to Microsoft for analysis. This is selected by default, and reports the erroneously quarantined message to Microsoft as a false positive. Please note that future messages from the sender may still be quarantined.
7. When you are done reviewing your messages, click Release messages.