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By , IT support technician

Quick Answer

Go to the Send / Receive tab and click Work Offline, open your Outbox, and delete or move any stuck email with a large attachment. Click Work Offline again to reconnect and the queue sends.

How to Fix "Cannot Send Email" Errors in Outlook ​

Applies to: Microsoft Outlook 365 (classic and new Outlook), Outlook on the web, Exchange
Article Type: Troubleshooting
Last Updated: 2026-07-29

Summary ​

Outgoing mail fails for a handful of predictable reasons: a message jammed in the Outbox, an attachment over the size limit, offline mode left switched on, or a permissions error when sending from a shared mailbox. This article works through each one in the order most likely to fix it.

Symptoms ​

  • Emails sit in the Outbox folder and never move to Sent Items.
  • The message count next to Outbox keeps climbing as new mail piles up behind the stuck one.
  • Outlook displays Sending' reported error (0x8004010F) or a similar send error.
  • A bounce-back message arrives saying the message could not be delivered.
  • Sending from a shared mailbox fails with You do not have permission to send on behalf of the specified user.

Prerequisites ​

  • Verify the email address of the recipient is correct.
  • Ensure the attachment is under 25 MB (the standard Microsoft 365 limit).

Instructions ​

1. Clear Emails Stuck in the Outbox ​

One oversized email at the front of the queue blocks every message behind it. Outlook keeps retrying that message and never reaches the rest.

  1. Go to the Send / Receive tab and click Work Offline. This stops Outlook from trying to transmit, which is what locks the message in place.
  2. Click Outbox in the folder list on the left.
  3. Right-click the oldest message in the list and select Delete, or drag it to Drafts if you want to keep and edit it.
  4. Click Work Offline again to reconnect. The button stops appearing highlighted.
  5. Watch the Outbox — the remaining messages move to Sent Items within a few seconds.

Why the message would not delete before: Outlook holds a lock on any item it is actively transmitting. Switching to offline mode releases that lock, which is why the delete works now and did not a moment ago.

2. Check Whether Outlook Is Working Offline ​

Offline mode queues mail without sending it, and it gets left switched on by accident.

  1. Look at the status bar at the very bottom of the Outlook window.
  2. If it reads Working Offline or Disconnected, go to the Send / Receive tab.
  3. Click Work Offline to turn it off.
  4. The status bar changes to Connected to: Microsoft Exchange and queued mail sends.

3. Verify the "From" Address ​

If you manage multiple inboxes through delegated access, you might be sending from the wrong one.

  1. In the New Email window, look at the From field above To.
  2. Confirm it matches an account you have permission to send from.
  3. If you do not see the From field, open the Options tab and click From to reveal it.

Note: Seeing a mailbox in your folder list does not mean you can send from it. Viewing and sending are separate permissions — see step 5 of the troubleshooting table if this is your error.

4. Send a Large Attachment as a Link Instead ​

Attachments over 25 MB are rejected by the server, and the message sits in the Outbox retrying forever.

  1. Delete the stuck message using step 1 above.
  2. Upload the file to OneDrive.
  3. In a new email, go to the Insert tab, click Link, and choose the file from your OneDrive.
  4. The recipient gets a link with no size limit, and the message sends immediately.

5. Check App Passwords (Gmail/Yahoo users) ​

If you added a personal account to Outlook, your normal password will not work once two-step verification is on.

  1. Sign in to your email provider's website.
  2. Go to Security Settings > 2-Step Verification.
  3. Generate an App Password.
  4. Update your Outlook password with this new 16-character code.

Troubleshooting ​

DANGER

If you receive a bounce-back (NDR) message, read the code in it before retrying. 550 means the recipient address does not exist — resending will fail the same way. Correct the address first.

Symptom / ErrorPotential CauseSolution
Emails sit in the OutboxOffline mode is onFollow step 2 to turn off Work Offline.
Message will not delete from the OutboxOutlook is locking it mid-sendSwitch to Work Offline first, then delete. See step 1.
The message exceeds the maximum sizeAttachment over 25 MBSend it as a OneDrive link instead. See step 4.
You do not have permission to send on behalf of the specified userMissing Send As rightsTell IT: "Please grant my account Send As permission on the shared mailbox."
Everything sends from webmail but not the appCorrupt local profile or cached credentialsSign in at outlook.office.com to confirm the account works, then restart Outlook.
Mail sends but recipients never receive itMessage blocked after sendingCheck your Sent Items — if it is there, the block is server-side. Report it to IT with the recipient address and send time.