1. Messages were manually deleted either due to accidental deletion or were moved or deleted due to your account filter settings.

2. Messages were deleted due to account compromise issues where a hacker might have gained access to your account and deleted the emails.

3. If you are using an e-mail client or a third party spam filtering software to access your account. This often happens when using the Pop3 protocol to retrieve your mail and the option “Always leave a copy of the message on the server” is not selected.

4. The account was marked as “Inactive." Accounts become inactive if you don’t sign in for more than 365 days or within the first 10 days after signing up for an account.

5. If the feature Conversation Threading feature is turned on.  You can click here to know more about Conversation Threading

6. If you use Sweep to schedule a cleanup. Click here to see what Sweep can do.

7. If you rename your account to @Outlook.com, a prompt would have asked you – ‘Where do you want to save your emails?’ You will have the option to create a new folder for your old account or save the emails in the Inbox of your renamed account.

8. If you were accessing your emails on a mail client such as Windows Live Mail or through your mobile device, but didn’t log into your web account for over 365 days, you may lose emails due to inactivity in web account.