First, work out where the problem is

Before changing anything, open your mailbox in a web browser at outlook.office.com and sign in with the same account.

  • The email is there in the browser but not in Outlook on your PC: the problem is with Outlook on that computer. Start at step 2.
  • The email is missing in the browser too: the problem is with the mailbox or with delivery, not with Outlook. Check step 1, then look at mailbox size (step 7) and your spam and quarantine folders.
  • Only one device is wrong, for example your phone: fix that device. Removing and re-adding the account on the phone usually sorts it.

1. Check whether Microsoft has a problem

Sometimes it is not you. If you are an administrator, the Microsoft 365 admin centre has a Service health page that lists current issues. If you are not, ask whoever looks after your IT, or check whether colleagues are seeing the same thing. If Microsoft has an incident, waiting is the fix.

2. Make sure Outlook is not offline

In classic Outlook for Windows, look at the bottom of the window. If it says Working Offline or Disconnected:

  1. Click the Send / Receive tab.
  2. Click Work Offline once to switch it off. The button is highlighted when offline mode is on.
  3. Click Send/Receive All Folders.

Also check the computer is actually online by opening a website. Hotel, café and some office guest Wi-Fi can block email while web pages still load.

3. Restart Outlook properly

Close Outlook, then open Task Manager (Ctrl + Shift + Esc) and make sure no Outlook process is still running. End it if it is. Then restart the computer if it has not been restarted for a while. It sounds too simple, but it fixes a surprising number of sync problems.

4. Update Office

In Outlook, go to File, then Office Account, then Update Options, then Update Now. Old builds have known sync bugs that are already fixed. Restart Outlook after the update.

5. Clear and resync one folder

If only one folder is wrong, such as the Inbox or Sent Items, you can make classic Outlook download it again without touching anything else:

  1. Right-click the folder and choose Properties.
  2. On the General tab, click Clear Offline Items, then OK.
  3. Click Send / Receive, then Update Folder.

This only clears the copy on your computer. The emails are still safe in the mailbox online.

6. Check how much mail Outlook keeps offline

Classic Outlook may only keep the last few months on the computer. Older emails then appear to be "missing" when they are just not downloaded. Go to File, Account Settings, Account Settings, double-click your account, and move the Download email for the past slider. Choose All if you want everything on the PC and there is room on the drive.

7. Check the mailbox is not full

A full mailbox stops sending and then stops receiving. In Outlook, File shows the mailbox size under Mailbox Settings. Most Microsoft 365 business plans give a 50 GB mailbox. If you are near the limit, empty Deleted Items, delete large old attachments, or ask your administrator about an archive.

8. Repair Office

  1. Close all Office apps.
  2. In Windows, open Settings, then Apps, then Installed apps.
  3. Find Microsoft 365 (or Microsoft Office), click the three dots or select it, and choose Modify.
  4. Try Quick Repair first. If the problem comes back, run Online Repair, which reinstalls Office and takes longer.

9. Rebuild the Outlook data file

Classic Outlook keeps a local copy of your Microsoft 365 mailbox in an .ost file. If that file is damaged, Outlook can show old or missing mail. Because everything is stored online, you can safely make Outlook build a new one:

  1. Close Outlook.
  2. Press Windows key + R, type %localappdata%\Microsoft\Outlook and press Enter.
  3. Find the .ost file named after your email address and rename it, for example by adding .old to the end.
  4. Open Outlook. It creates a fresh file and downloads the mailbox again. On a large mailbox this can take a while.

Only do this for Microsoft 365 or Exchange accounts. A .pst file is different: it can hold email that exists nowhere else, so never delete or rename a .pst file unless you know it is backed up.

10. Create a new Outlook profile

If nothing else works, a new profile starts Outlook fresh without touching the mailbox itself:

  1. Close Outlook and open the Windows Control Panel. Search for Mail and open Mail (Microsoft Outlook).
  2. Click Show Profiles, then Add, and give the profile a name.
  3. Enter your email address and let Outlook set it up.
  4. Choose Always use this profile and select the new one, then open Outlook.

Shared mailboxes you were given access to normally reappear by themselves. Signatures and some personal settings may need setting again, so note them before you start.

Using the new Outlook?

The new Outlook for Windows works more like the web version and does not use .ost files or profiles in the same way. If it will not sync, check the web version first, then sign out and back in through Settings and Accounts. You can switch back to classic Outlook with the toggle at the top of the window while you investigate.

When to get help

If mail is missing in the browser as well, if several people are affected at once, or if emails you send are bouncing back, the problem is likely to be with the mailbox, the licence or the domain records rather than one PC. That needs someone with admin access to look. RoamTech fixes Outlook and Microsoft 365 problems remotely for small businesses across the UK. See Business IT Support or remote IT support.