If an email is stuck in the Outlook Outbox, the quickest fix is to go to Send/Receive and select Work Offline so Outlook stops retrying, then open the Outbox and either delete the message or drag it to Drafts and remove its attachment. Microsoft names a large attachment as the most likely cause, and being disconnected while you click Send as the second.
This guide covers the causes in the order you are most likely to meet them, and finishes with the settings and add-in checks for messages that keep returning.
Who this is for and what you will have at the end
This is for anyone using classic Outlook for Windows, with Microsoft 365 or another account, whose message sits in the Outbox, or whose mail only goes out when you press F9. It also helps the person in a small business who is asked why "email isn't going out" from a colleague's PC.
By the end you will have cleared the stuck message, sent it again, and checked the settings that cause repeat problems. The steps on Microsoft's support pages are written for classic Outlook; where a step is for a different version, this article says so.
Why messages get stuck in the Outbox
When you select Send, Outlook places the message in the Outbox folder and then hands it to the mail server. A message that stays there has not completed that hand-off. Microsoft's support article on stuck messages gives two causes:
- A large attachment. Microsoft describes this as the most likely cause.
- No connection when you clicked Send. The message waits until Outlook is connected again.
Microsoft's Learn troubleshooting content adds a third:
- Send immediately when connected is turned off. The message stays in the Outbox until you start a Send/Receive manually, for example by pressing F9.
Add-ins are a further possible cause. Microsoft's pages above do not list them for this symptom, but safe mode starts Outlook with add-ins turned off, so it is a cheap test (Step 5).
The table below maps the symptom to the section that fixes it.
| What you see | Most likely cause | Go to |
|---|---|---|
| One message stays in the Outbox, others send | Large attachment | Step 2 |
| Everything stays in the Outbox, Work Offline button is blue | Outlook is disconnected | Step 1 |
| Mail sends only after pressing F9 | Send immediately when connected is off | Step 4 |
| Sending worked until an add-in was installed | Add-in interference | Step 5 |
Before you start
- Note which message is stuck and its recipients, so you can resend it if you delete it.
- Check that the PC is online by opening a web page. If the PC has no internet, fix that first; the Outbox will clear once Outlook can connect.
Step 1: Check Work Offline
If Outlook is set to Work Offline, nothing sends.
- Select the Send/Receive tab.
- Look at the Work Offline button. If it is blue, Outlook is disconnected.
- Select it once. The button turns white, which means Outlook is online again.
- Select Send All.
This is the exact procedure Microsoft gives for sending when disconnected. If the messages leave the Outbox, you are done. If the Work Offline button turns blue again by itself, look at the network connection and at Step 5.
Step 2: Clear a message stuck on a large attachment
If one message refuses to leave while others are fine, handle it directly. Microsoft's steps are:
- Select Send/Receive > Work Offline to pause sending. This stops Outlook retrying while you work on the message.
- In the Navigation Pane, select Outbox.
- Choose one of these options:
- Delete the message. Select it and press Delete. Use this when you can recreate it.
- Remove the attachment. Drag the message to Drafts, double-click it to open it, select the attachment and press Delete.
- If Outlook says it is transmitting the message, close Outlook and wait a moment for it to exit. If it will not close, press Ctrl+Alt+Delete, open Task Manager, go to the Processes tab, select outlook.exe and select End Process.
- Reopen Outlook and repeat steps 2 and 3 if the message is still there.
- Select Send/Receive > Work Offline again to turn it off and return to online mode.
After this, send the file another way: attach a smaller file, or upload it to OneDrive and share a link instead. Microsoft recommends the OneDrive link approach for large files on its Outlook.com sending page.
How big is too big?
The limit depends on the account and the client. For Microsoft 365 mailboxes, Microsoft's Exchange Online limits page (updated October 2026) lists these defaults:
| Client or setting | Limit |
|---|---|
| Message size, Outlook | 150 MB |
| Message size, Outlook on the web | 112 MB |
| Message size, Outlook for iOS and Android | 33 MB |
| File attachment size, Outlook | 150 MB |
| Number of file attachments | 250 |
The page states that message size limits include attachments, that administrators can configure them up, down or per user, and that some email clients may have lower limits. A message above the limit is not delivered and the sender receives a non-delivery report (NDR). That is different from a message that sits in the Outbox, but the same advice applies: reduce the size. Also remember the recipient's own mail system may reject large messages even when yours accepts them.
For Outlook.com accounts, Microsoft's support page says to remove an attachment over 25 MB, and either attach a smaller file or put the file on OneDrive and attach a link.
Step 3: Send again and confirm
Once Work Offline is off:
- Select Send/Receive > Send/Receive All Folders, or press F9.
- Watch the Outbox. The message should disappear within moments, and a copy should appear in Sent Items.
- If you deleted the message, create a new one with the smaller attachment or the OneDrive link.
If the Outbox is empty and the Sent Items copy exists, the send worked. If you use an Outlook.com account, Microsoft also suggests checking whether the stuck message is in Drafts or the Outbox, and emptying the Junk Email folder if the mailbox is full, then trying again.
Step 4: Fix messages that wait for a manual Send/Receive
If every message sits in the Outbox until you press F9 or select Send/Receive, the cause is a setting. Microsoft's Learn article on this symptom says it happens when Send immediately when connected is not enabled.
- In Outlook, select File > Options.
- Select Advanced.
- In the Send and receive section, select Send immediately when connected.
- Select OK.
If the option is grayed out, Group Policy is managing it. You cannot change it yourself; ask your administrator to remove or change the policy.
For administrators: the registry value behind the setting
Microsoft documents that this option maps to a registry value, so it can be set by an administrator or a policy. The value is a DWORD named Send Mail Immediately, and a value of 0 turns the behavior off.
Key: HKEY_CURRENT_USER\Software\Microsoft\Office\<x.0>\Outlook\Options\Mail
Policy: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\<x.0>\Outlook\Options\Mail
DWORD: Send Mail Immediately
Value: 0 (messages wait for a manual Send/Receive)
In these paths, <x.0> is the Outlook version. Microsoft states that 16.0 covers Outlook 2016, Outlook 2019 and Outlook for Microsoft 365. Prefer the Outlook option or your normal policy tooling over hand-editing the registry, and back up before any registry change.
Step 5: Test for an add-in problem with safe mode
If the Outbox keeps filling after the steps above, test whether an add-in is responsible. Safe mode starts Outlook with add-ins turned off.
- Close Outlook.
- Right-click the Start button and select Run.
- Enter the following and select OK:
Outlook.exe /safe
- If Windows cannot find the command, enter the full path to Outlook using the Microsoft Office folder for your version, under Program Files.
- Choose your profile if asked, then try sending a short test message to yourself.
If the test message sends in safe mode but not normally, an add-in is the likely cause. Disable your add-ins one at a time in normal mode, testing after each, until the problem returns, and prefer disabling over removing. Microsoft's safe mode page links to a separate article that covers disabling add-ins; follow that article for the exact steps on your Outlook build.
If the message does not send even in safe mode, the cause is elsewhere: the account, the mailbox, the attachment or the connection.
Verification
Use this short test to confirm everything is fixed.
- Confirm the Work Offline button on the Send/Receive tab is white.
- Confirm Send immediately when connected is selected under File > Options > Advanced.
- Send a message with no attachment to yourself.
- Confirm it leaves the Outbox at once and arrives in the Inbox.
- Send a message with a modest attachment, and a link to a OneDrive file, and confirm both leave the Outbox.
If all three pass, the problem was the offline state, the setting or the oversized attachment, and the Outbox should stay clear.
Troubleshooting
Outlook says it is transmitting the message and will not let you delete it
The message is mid-send. Close Outlook and wait a moment. If it does not exit, open Task Manager, select outlook.exe on the Processes tab and select End Process, then restart Outlook and delete or move the message. This is the same fallback Microsoft documents.
Messages stay in the Outbox with an Exchange account in Cached Exchange mode
Microsoft's older Outlook guide for Outlook 2007 to 2013 describes a case where messages do not send immediately for an Exchange account in Cached Exchange mode and the problem does not occur in safe mode. The cause there is a LoadTransportProviders DWORD with a value of 1 under the Outlook Preferences registry key, and the permanent fix is to delete that value. This applies to those older versions; on current Microsoft 365 builds, start with the earlier steps instead.
Outlook hangs while loading the profile
A Microsoft page from 2019 describes a service issue that was fixed on June 25, 2019 and gives a workaround for a hang at profile load. It is unlikely to apply to a current build, so skip it unless the symptom matches exactly.
Still stuck after all steps
If messages stay stuck after the steps above, ask your Microsoft 365 administrator to look at the mailbox and at any third-party add-ins or security software that scans outgoing mail. If your issue is a sign-in prompt rather than a stuck message, see Stop Outlook asking for your password again and again. For problems with Outlook finding the right server, see Outlook Autodiscover troubleshooting for Microsoft 365.
Checklist
- Work Offline is off (the button is white) and you have selected Send All.
- You removed or deleted the message that was stuck on a large attachment, and shared the file through OneDrive instead.
- Send immediately when connected is selected under File > Options > Advanced.
- You tested in safe mode with
Outlook.exe /safeand disabled any add-in that blocked sending. - A test message and a message with a modest attachment both left the Outbox.
References
- Fix messages that are stuck in the Outbox - Microsoft Support
- Outlook email stuck - Microsoft Support
- Email stays in Outbox until manually send or receive - Microsoft Learn
- Exchange Online limits - Microsoft Learn
- Open Outlook in safe mode - Microsoft Support
This guide was researched and drafted with AI assistance and checked against the vendor documentation listed in its References. Read the maintained version and related guides on EpiFive.
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