If your Outlook.com account is set up in Outlook 2013 or Outlook 2016 and it has stopped syncing, you need to remove the account from Outlook and use auto account setup to reconnect the account as a Microsoft Exchange account.  You will no longer be able to use an Exchange ActiveSync  (EAS) account type to access Outlook.com email from  Outlook. (EAS will continue to work with smartphones.)
Any appointments, contacts, tasks, or notes added after it stopped syncing will be lost when you remove the account from Outlook and these need to be moved or exported before doing removing the EAS account from Outlook.
If you remove the account and don't export the contents to a pst file first, you will not be able to recover any missing appointments or contacts.
I recommend you add the account to your profile using auto account setup then compare the calendars, contacts, tasks, and notes to see if anything needs to be moved to the server before removing the EAS account from your profile.
To switch to a Microsoft Exchange account type
If you are not using an Outlook.com (including Hotmail or Live.com addresses), you'll need to add an Outlook.com alias to your account first and set it as the Primary account. At this time, you can't set up the account with a non-Microsoft account. See Add an Alias to an Outlook.com Account for more information.
- Open File and click Add .
- Type in your email address then click Next. You can leave the name and password fields blank for Outlook.com and Office 365 Exchange accounts. The display name is picked up from the Microsoft or Active Directory account and Outlook will ask for your password if it's not stored in the Windows Credential Manager.
- Restart Outlook after the account is added. The newly added account may have a (1) after the name. This will go away after the EAS account is removed.
- Switch to the calendar module and display both Outlook.com account calendars side-by-side and browse a few months. If any appointments are missing from the Exchange calendar, drag them to the Exchange calendar. You can also use instant Search on the old calendar (make sure the search is for 'current Folder') and use this as the search term: datemodified:>1/21/2017
- Repeat with Contacts (use any view except People to get the item count), Tasks, and Notes (if used). Use Instant Search to filter the view to newly created items.
- After you moved the unsynced items over, remove the EAS account in File, Account Settings by selecting the account, then click Remove .
- Restart Outlook and the (1) should be removed from the account name and folder list.
