There are several reasons why Microsoft Outlook won't send mail and before you can fix the problem, you need to identify the cause.

Windows 10 users who are having problems sending email after upgrading their computer, see Outlook Error Ox800CCC13 in Windows 10.
iCloud users, see "After Installing iCloud, Mail Won't Send?". You may need to change your SMTP server name.

Possibly the most common cause is the message is too large to send because you added a huge attachment to a message. If this is the problem, see Delete large messages from the Outbox if you can't delete the stuck message. Instructions for Outlook 2011 for Mac are at Delete Messages Stuck in Outlook 2011 Outbox.

If you are using iCloud and messages get stuck, see Outlook and iCloud 2: What you need to know for the latest information.

If the message is not too large to send, other possible causes include:

1. An add-in that marks items read if you view the Outbox
2. Wrong password or not authenticating properly with the mail server
3. Outlook or the mail server is offline
4. No default email account is set
5. Corrupt Send and Receive Settings
6. Using a program that accesses pst or ost data, such as Desktop Search, Lync, etc
7. An antispam or antivirus scanner is scanning outgoing mail

Troubleshooting

First, check the Send/Receive progress dialog for error codes. Browse to Tools, Send/Receive, Send/Receive settings, Show progress and click on the error tab. The error will resemble this, with a different error number and possibly a better explanation than "unknown error"

Task '<account> - Sending' reported error (0x80040119) : 'Unknown Error 0x80040119

See What does this error mean? for the reasons for some of the error codes. (If you need the text of the error code, you can select any error and use Ctrl+C to copy it.)

An add-in marks items read if you view the Outbox

Symptom: you look at the mail in the Outbox and it's not bolded.
Quick Fix: Stop looking in the Outbox.
See Outlook: After viewing the Outbox, e-mail won't send for applications that cause messages in the Outbox to be marked as read.

Wrong or bad password, or not authenticating properly with the mail server

Verify your password works by logging into your account using web access.

Check your account settings:

1. Go to Tools menu (Outlook 2007) or File tab (Outlook 2010) then Account Settings, Email.
2. Double click on the account then click More Settings.
3. Check your settings on the Outgoing server tab. Many email providers now require a password to send email. If you aren't sure, enabling authentication before sending usually doesn't cause problems, but you should verify the proper settings with your email provider.
4. Do not enable SPA unless your mail server requires it.
5. Verify the port number is correct on the More Settings, Advanced tab. Many email providers use port 587 for SMTP.

In Outlook 2003 and older, go to Tools, E-mail accounts, View or Change existing accounts. Double click on the account and click More Settings, then select the Advanced tab.

Outlook or the mail server is offline

Is Outlook online? The online status is in the Status bar.

Is the mail server online? Outlook's status bar may say 'Trying to Connect' if the mail server is down.

No default email account is set

Symptoms include being able to reply but not send new mail.
This may happen when you use a preconfigured script provided by the mail server administrator to set up your account.
In Outlook 2010, go to File, Account Settings, Email. Is an set as default?
In Outlook 007, you'll check for a default account in Tools, Account Settings, Email. In Outlook 2003 and older, go to Tools, E-mail accounts, View or Change existing accounts. Verify an account is set as default.

The SRS file is corrupt

Outlook saves your Send and Receive customizations in a file with the extension *.SRS. If this file is corrupt, the Send & Receive Settings dialog won't open and you may receive send & receive errors. Find and delete or rename the SRS file for your profile.

In Windows 7, 8 and Vista the SRS file is at C:\Users\username\AppData\Roaming\Microsoft\Outlook\. In Windows XP, the SRS file is found at C:\Documents and Settings\%username%\Application Data\Microsoft\Outlook\. In either case, you can type or paste %appdata%\microsoft\outlook in the address bar of Windows Explorer, then press Enter, to jump to the folder.

Using a program that accesses the .pst or .ost data

Symptom: Mail sends at first, then hangs in mailbox
Send and Receive error code is: Task '<account> - Sending' reported error (0x80040119) : 'Unknown Error 0x80040119.

Solutions include:

Use Scanpst to repair the pst file, or delete the ost file and let Outlook rebuild it. See Error message when you try to read or to delete email or when you click "Send/Receive" in Outlook: "An unknown error has occurred" or "Messaging interface has caused an unknown error" (MSKB) for more information.

Restarting Outlook (after verifying it closed completely in Task manager) may allow the mail to send.

Otherwise:

Reboot and disable the problem program before starting Outlook.

If it continues to cause problems (such as sending works for awhile after restarting Outlook, then stops), uninstall the software.

An antispam or antivirus scanner is scanning outgoing mail

Norton and AVG antispam plugins are known to cause problems, but there are others. Any antivirus that uses an Outlook addin can cause issues with your email.

Solution: Disable Email scanning in the software.

It may help to the timeout (in your email account's More Settings, Advanced tab.) Otherwise, check your antispam or antivirus program's website for an update or possible workarounds. Note that we do not recommend scanning email for viruses as any virus will be picked up if you try to open an infected message (which you know not to do :)).

If none of the above are the cause and the account settings look okay, turn on SMTP logging.

To turn on logging in Outlook 2010, go to File, Options, Advanced. Enable Troubleshooting logging is near the bottom, in the Other section. I n Outlook 2007 and Outlook 2003, go to Tools, Options, Other, Advanced and add a check to the box beside Enable Logging (troubleshooting).

Because logging can cause Outlook to be unstable, initiate a send and receive and once you've reproduced the error, disable logging. The log files are stored in your temp folder. Type %temp% in the address bar of Windows Explorer and press Enter to open the Temp folder. The log files will have the extension .log and may be in an Outlook Logging subfolder.

How to enable transport logging in Outlook (MSKB)

Hidden Messages that won't Send

When you have a hidden message that won't send, it's a stuck read receipt. They are stored as hidden messages at the root level of your mailbox.

There are two methods that can be used to delete them:

1. Change the SMTP server for the account to a SMTP that can send the receipt. After the read receipt is sent, change the SMTP back to the one you normally use for the account after they are sent.

2. Delete the hidden messages using MFCMAPI or OutlookSpy. Because MFCMAPI does not need to be installed, it's our choice for most users. (We do not recommend using MDBVue as it is outdated and was replaced by MFCMAPI.)

We have complete instructions for deleting stuck read receipts using MFCMAPI at Delete Read Receipts using MFCMAPI.