You hit Send, the message slides into the Outbox, and then nothing. No "Sent" confirmation, no error you can act on, just an email sitting there while replies pile up. Receiving might still work fine, which makes the silence on the sending side even more confusing.
The good news: stuck outgoing mail in Outlook almost always traces back to a short list of fixable causes. A lost connection, an attachment that is too large, a setting toggled off, or a password that quietly went stale.
Work through the fixes below in order. They are arranged quickest and most common first, so the early steps clear the majority of cases before you ever touch server settings or repair tools. Where a step differs between classic Outlook for Windows, new Outlook, Outlook.com on the web, or the mobile apps, that is called out directly.
Check Your Connection and Turn Off Work Offline
If Outlook cannot reach the mail server, your message simply waits in the Outbox. Start by confirming the basics.
Open a browser and load a site like microsoft.com or bing.com to confirm the internet is actually working. On mobile, make sure Wi-Fi or cellular data is connected and the device has free storage.
In classic Outlook for Windows, look at the status bar along the bottom of the window. If it reads "Working Offline," "Disconnected," or "Trying to connect," Outlook is not online.
- 1.Go to the Send / Receive tab.
- 2.In the Preferences group, click Work Offline to toggle it off (the highlighted button returns to normal), which reconnects Outlook.
- 3.Click Send/Receive All Folders, or press F9, to push the queued messages.
Once you are back online, your stuck mail often goes out on its own.
Force a Manual Sync
In new Outlook for Windows, an account that has not synced can leave mail unsent. Go to the View tab and click Sync, then watch the status message at the bottom of the message list.
One important limitation: manual Sync does not work for Gmail, Yahoo, or iCloud accounts in new Outlook, so do not expect it to clear a stuck message for those. For desktop clients tied to an Outlook.com account, running Send/Receive > Send/Receive All Folders forces a fresh sync with the servers.
Remove an Oversized Attachment
A large attachment that exceeds the server's size limit is the single most common reason a message gets stuck in the Outbox, and it often shows no obvious error. Outlook.com caps attachments at 25 MB; classic Outlook guidance commonly advises removing attachments larger than about 20 MB. The true ceiling is set by your mail provider, so other servers may reject even smaller files.
In classic Outlook for Windows, take the account offline first so it stops trying to send while you edit.
- 1.Click Send / Receive > Work Offline.
- 2.In the navigation pane, click Outbox.
- 3.Open the stuck message (oldest first). If it will not open from the Outbox, drag it to the Drafts folder first, then double-click it.
- 4.Select the attachment and press Delete.
- 5.Attach a smaller file, or upload the file to OneDrive and insert a link instead, then send.
- 6.Close and restart Outlook, then click Send / Receive > Work Offline again to reconnect.
On Outlook.com, open the stuck email from Drafts or Outbox, delete any attachment over 25 MB, then attach a smaller file or insert a OneDrive link and resend.
Release a Message You Accidentally Opened
A message waiting in the Outbox should appear in italics, meaning it is unread and still trying to send. If you open or preview it, it can switch to non-italic and stop sending entirely.
The fix is to re-queue it cleanly:
- 1.In the navigation pane, click Outbox and check whether the message is in italics.
- 2.Drag the message from Outbox to the Drafts folder.
- 3.Open it from Drafts and click Send so it queues correctly.
If a stuck message refuses to release while you edit or delete it, go offline first (Send/Receive > Work Offline); otherwise Outlook may keep trying to send and lock it. If it still will not let go, drag it from the Outbox to Drafts.
Re-enable Send Immediately When Connected
In classic Outlook for Windows, if this option is off, messages sit in the Outbox until you manually run a Send/Receive (F9).
- 1.On the File tab, select Options.
- 2.In the Outlook Options dialog box, select Advanced.
- 3.In the Send and receive section, turn on (check) Send immediately when connected.
- 4.Select OK.
In Outlook 2007 and 2003, the path is Tools menu > Options > Mail Setup tab > enable Send immediately when connected > OK. If the checkbox is grayed out, it is being enforced by Group Policy and only an administrator can remove that policy.
Update Your Password
After a recent password change, Outlook can no longer authenticate to send. Update it to the exact password that works when you sign in on the provider's website.
In classic Outlook, go to File > Account Settings > Account Settings, select the affected account, choose Change, and re-enter the current password, then run Send/Receive to test. In new Outlook, select the icon next to the email account name, click Continue on the pop-up, and enter the new password.
If two-step verification is on, or normal password sign-in fails, generate and use an app password from your provider instead of your regular password.
Clear Space in a Full Mailbox
When your mailbox or cloud storage is full, you cannot send or receive. Worse, messages people send you while you are over quota bounce back and are not recoverable even after you free space, so act promptly.
On Outlook.com, check your storage, and if it is full, clear space; for example, right-click the Junk Email folder and select Empty folder. New accounts also start with a low sending quota that rises automatically as the account builds credibility, so if you just created it, wait and retry.
Enable IMAP/POP and Verify Server Settings
Outlook.com has POP and IMAP access turned off by default, which blocks third-party and desktop clients from sending until you switch it on. In Outlook.com on the web, go to Settings > Mail > Forwarding and IMAP and turn on POP/IMAP access.
Then confirm the correct Outlook.com server settings in your client:
- Incoming IMAP: server outlook.office365.com, port 993, SSL/TLS encryption.
- Incoming POP: server outlook.office365.com, port 995, SSL/TLS encryption.
- Outgoing SMTP: server smtp-mail.outlook.com, port 587, STARTTLS encryption.
Use your full email address as the username. Outlook.com requires modern authentication (OAuth2) and does not support legacy basic authentication, so your third-party app must support OAuth2. If normal sign-in fails or two-step verification is on, use an app password.
For any POP or IMAP account in classic Outlook, also confirm outgoing authentication is enabled:
- 1.Go to File > Info > Account Settings > Account Settings, select the account, and click Change.
- 2.Click More Settings, then open the Outgoing Server tab.
- 3.Check "My outgoing server (SMTP) requires authentication" and select "Use same settings as my incoming mail server."
- 4.Open the Advanced tab and confirm the outgoing (SMTP) port and encryption match your provider (common SMTP ports are 587 with STARTTLS, 465 with SSL, or 25).
- 5.Click OK, run Test Account Settings, then Finish, and send a test message.
Remove Your Signature
On Outlook.com, a problematic email signature can block a specific message from sending. If one particular email refuses to go while others work, remove your signature and try sending again.
Set a Default Account and Rule Out Add-ins
In classic Outlook, if nothing sends at all, you may have no default account set, which is common after admin-scripted setup. Go to File > Account Settings, select the correct account, and choose Set as Default.
To rule out a faulty add-in, start Outlook without them: press the Windows key, type outlook.exe /safe and press Enter. If the stuck message sends in Safe Mode, an add-in is the cause. Go to File > Options > Add-ins, disable them, and re-enable one at a time to find the culprit.
Whitelist Outlook in Your Antivirus or Firewall
Some security software scans outgoing mail or blocks SMTP and IMAP ports, which halts sending. Temporarily disable email scanning (or whitelist Outlook and the SMTP/IMAP ports) to test, then re-test sending.
Treat disabling protection as a diagnostic only. Re-enable it afterward and whitelist Outlook and the relevant ports rather than leaving your security off.
Remove a Bad Mobile Device Partnership
A misbehaving phone or tablet partnership can break sending and receiving across your whole mailbox. Removing the device often restores it.
- 1.In Outlook.com or current Outlook on the web, go to Settings > General > Mobile devices.
- 2.Select the problematic device and click the delete/Remove icon, then click Save.
- 3.In new Outlook for Windows, the path is Settings > Accounts > Mobile devices.
- 4.Power the mobile device completely off, wait a few seconds, turn it back on, and let the mailbox resync.
If it still will not resync, fully remove and re-add the account on the device.
Resync or Reinstall the Mobile App
On iOS and Android, first confirm the device has free storage and a working connection. For an Outlook.com or Microsoft 365 account that will not send, remove the device sync partnership, then remove and re-add the account in the app.
If the app crashes on open, clear the browser cache on the device, then uninstall and reinstall the Outlook app. One caution: if you close the app before a send completes, the unsent message may be silently moved to Drafts rather than actually sent, so check Drafts and resend.
Repair a Corrupted Profile, Installation, or Data File
If sending only works under a fresh profile, your Outlook profile may be corrupt, and creating a new one resolves it. If you suspect a damaged Office installation, repair it through the Windows Apps or Programs list. If a .pst data file is corrupt, run the Inbox Repair Tool (scanpst) on that file.
If Outlook will not close to apply any of these fixes, open Task Manager (Ctrl+Alt+Delete), end the outlook.exe process, then restart.
Check for a Bounce or Non-Delivery Report
If a message left your Outbox but never reached the recipient, the cause may be on their end: an unknown address, a recipient block, or a full recipient mailbox. These produce a non-delivery report (NDR). Verify the recipient address and read the NDR carefully, as it names the specific error.
Frequently Asked Questions
Why is my email stuck in the Outbox with no error message?
The most common silent cause is an attachment that exceeds the server's size limit; the message just sits there with no obvious warning. Outlook.com caps attachments at 25 MB, and other servers may reject smaller files. A lost connection or Work Offline mode in classic Outlook produces the same quiet stall.
Why won't a message in my Outbox send even though Outlook is online?
A sendable message appears in italics. If you opened or previewed it, it can switch to non-italic and stop trying to send. Drag it from the Outbox to Drafts, open it from there, and click Send to re-queue it correctly.
Do I need to enable anything for Outlook.com to work in a third-party email app?
Yes. POP and IMAP access is off by default in Outlook.com, under Settings > Mail > Forwarding and IMAP, and must be turned on. The account also requires modern authentication (OAuth2) and does not support legacy basic auth, so your app must support OAuth2.
Why does sending fail right after I changed my password?
Outlook is still using the old password and can no longer authenticate. Update it to the exact password that works on the provider's website. If two-step verification is on or normal sign-in is rejected, generate and use an app password instead.
Why is the "Send immediately when connected" checkbox grayed out?
That setting is being enforced by Group Policy, which a regular user cannot change. An administrator must remove the policy before the option becomes editable.
I freed up mailbox space, so why are some messages still missing?
When your mailbox was full, any messages sent to you during that time were bounced back to the sender and are not recoverable, even after you clear space. That is why it is important to keep storage from filling up in the first place.
