When Outlook is open and looks fine but mail isn't flowing, the issue is almost always the connection to the server.

1. Check the status bar

Look at the bottom of Outlook. The status line tells you:

  • Connected to: Microsoft Exchange — good
  • Trying to connect — Outlook can't reach the server
  • Disconnected — confirmed offline
  • Working Offline — you (or something) toggled offline mode

2. Work Offline toggle

Surprisingly common. Send/Receive tab → Work Offline button. If it's highlighted/active, click it to turn off.

Outlook will reconnect and try to send queued mail.

3. Check the Outbox

Click Outbox in the folder list. Mail sitting there hasn't been sent yet. Possible reasons:

  • A large attachment is being uploaded — wait
  • Recipient address is wrong (Outlook will move it to Drafts with an error after retrying)
  • The mail server rejected it (check the error message on the item)

If you have a stuck message and you don't want to send it, open it → Delete or move to Drafts.

4. Send/Receive manually

Press F9 or Send/Receive tab → Send/Receive All Folders. Forces an immediate sync.

If nothing happens, the connection is the issue. If you see "Send/Receive Errors" appear, look at the error code.

5. Verify your network works

If you can't load any website, Outlook can't reach the server either. Fix the network first ([Wi-Fi connected but no internet](/kb/no-internet-but-wifi-connected)).

If browsing works but Outlook says "trying to connect":

  • Sign in to outlook.office.com in a browser
  • If web mail works, the issue is the Outlook desktop client, not your account or the server

6. Restart Outlook

Close completely (check Task Manager that no OUTLOOK.EXE is running) → reopen.

7. Check the data file

A corrupt OST file can stop sync silently. To rebuild:

  1. Close Outlook
  2. Navigate to %localappdata%\Microsoft\Outlook
  3. Rename your OST (e.g. add .old)
  4. Reopen Outlook — it builds a new one from the server

OK for Exchange/M365 accounts; for IMAP accounts, you may lose drafts.

8. Cached Exchange Mode

Outlook normally syncs your mailbox to a local cache for speed. If the cache is broken:

  • File → Account Settings → Account Settings → double-click your account
  • Toggle Use Cached Exchange Mode off
  • OK, restart Outlook — you're now connected directly to the server
  • Toggle back on after to rebuild the cache

9. Send/Receive errors specifically

Common codes:

  • 0x800CCC0E — server connection failed; check network and credentials
  • 0x80004005 — generic; usually password expired
  • 0x8004010F — OST file inaccessible
  • 0x80040115 — server not found; check Wi-Fi
  • 0x80042108 — can't connect to POP3 server; check incoming server settings

Each has its own KB article on the Microsoft site.

10. Quota check

If you've hit your mailbox quota, send stops working until you free up space:

  • Outlook bottom-right shows "X% used" if near limit
  • Delete large attachments from your Sent Items
  • Empty Deleted Items folder (which doesn't actually free space until emptied)
  • Archive old mail

When to ticket

Open a ticket if:

  • Outlook web works but desktop doesn't, after repair
  • Send/Receive errors persist after credentials/network checks
  • Your mailbox is at quota and you can't reduce it

Include the exact error code and your Outlook version (File → Office Account).