Outlook Not Sending Emails: 8 Fixes That Work

If Outlook is not sending emails, the message is almost always stuck in the Outbox. Open the Outbox folder, find the unsent message, and check the most common causes: Outlook is in Offline mode, the Send/Receive button has not been pressed, your password needs to be re-entered, an attachment is too large, or your SMTP server settings are wrong. Most fixes take less than two minutes once you know which one to apply.

When Outlook will not send emails, the message usually sits in the Outbox without an error popup. The fix depends on which underlying issue is blocking the send. This guide walks through the eight most common causes in order of how often they happen, with steps for classic Outlook, new Outlook, Outlook on the web, and mobile.

Quick Diagnostic: Where Is the Email?

Before troubleshooting, find the unsent message:

1. Open Outlook and click Outbox in the folder pane (in classic Outlook) or Drafts (new Outlook and web).
2. If the message is there in regular text, it has not been sent yet but has not failed - try clicking Send/Receive or Send.
3. If the message is in italic text, it is queued for sending.
4. If you see a red exclamation mark, Outlook tried and failed - hover over the icon for the error.

This tells you whether the problem is "Outlook is not trying to send" or "Outlook is trying but failing".

Fix 1: Turn Off Work Offline Mode

The single most common cause. In Outlook, Work Offline mode stops all sending and receiving, often turned on by accident.

Classic Outlook (Desktop)

1. Click the Send/Receive tab in the ribbon.
2. Look at the Work Offline button.
3. If it is highlighted (blue background), click it to turn off offline mode.
4. Click Send/Receive All Folders to push pending emails.

You can also check the status bar at the bottom of Outlook - it shows "Working Offline" if the mode is on.

New Outlook and Outlook on the Web

New Outlook does not have a separate Work Offline toggle - it relies on your network connection. If new Outlook will not send, check your internet connection and try restarting the app.

Fix 2: Press Send/Receive

Sometimes Outlook is online and the message is in the Outbox, but nothing has triggered a send. Force it:

1. Classic Outlook: press F9, or click Send/Receive All Folders in the Send/Receive tab.
2. New Outlook / Web: emails send automatically when you click Send. If a message is in Drafts, open it and click Send again.
3. Outlook Mobile: pull down on the inbox to refresh, which forces a send/receive cycle.

If the message moves out of the Outbox after this, you are done.

Fix 3: Re-Enter Your Password

If Outlook silently lost authentication to your mail server, sending will fail without an obvious error. This is common after a password change, MFA token expiry, or Microsoft 365 license renewal.

Classic Outlook (Desktop)

1. Click File > Account Settings > Account Settings.
2. Select your email account and click Repair.
3. Follow the prompts to re-authenticate.
4. If Repair does not work, click Change > More Settings > Outgoing Server and re-enter credentials.

New Outlook and Outlook on the Web

1. Click the gear icon > Accounts > Email accounts.
2. Find the account and click Manage.
3. Sign out, then sign back in.

Outlook Mobile

Settings > tap the account name > Reset Account (iOS) or Sign Out (Android), then add it back.

Fix 4: Clear a Stuck Outbox

A single oversized or corrupted email can clog the Outbox and stop all other outgoing messages.

1. Open the Outbox folder.
2. If the stuck email has a large attachment, open it, remove the attachment, and save it as a draft. Try to send without the attachment first. If it works, the attachment was the problem.
3. To delete a stuck email: right-click and choose Delete. If Outlook will not let you because it is mid-send, switch to Work Offline first, delete the email, then turn Work Offline back off.
4. If deletion fails entirely, restart Outlook in safe mode: hold Ctrl while opening Outlook, then try to delete from the Outbox.

After clearing the stuck email, send a fresh test message to confirm sending is restored.

Fix 5: Check Attachment Size Limits

Microsoft 365 caps attachments at 25 MB per email for most accounts. Your own mailbox may have a smaller send limit set by your admin (typically 10 MB for stricter setups).

1. If the email has a large attachment, save the file to OneDrive or SharePoint and send a sharing link instead.
2. For files larger than 25 MB, OneDrive or SharePoint is the only Microsoft-native option.
3. Check your organization's policy with your admin or by reviewing the Microsoft 365 admin center > Exchange admin > Mail Flow > Message size limits.

Fix 6: Verify SMTP Server Settings

If Outlook recently lost connection to your mail server or you added a new account, the outgoing server settings may be wrong.

Standard Microsoft 365 SMTP Settings

Setting	Value
SMTP server	smtp.office365.com
Port	587
Encryption	STARTTLS
Authentication	OAuth or username and password

To check or fix:

1. File > Account Settings > Account Settings.
2. Select the account, click Change.
3. Click More Settings > Outgoing Server.
4. Make sure My outgoing server (SMTP) requires authentication is checked.
5. Click the Advanced tab and confirm the port and encryption match the table above.

For Other Email Providers

• Gmail: smtp.gmail.com, port 587, STARTTLS. Requires an app password if you use 2FA.
• iCloud: smtp.mail.me.com, port 587, STARTTLS.
• Custom domain: ask your email host or check their docs.

Fix 7: Disable Antivirus Email Scanning

Some antivirus tools scan outgoing email and can block sends if they detect false positives. If Outlook started failing to send after an antivirus update, this is likely the cause.

1. Open your antivirus settings (Norton, McAfee, Avast, Bitdefender, etc.).
2. Find the Email scanning or Outbound mail scan option.
3. Disable it temporarily.
4. Try sending the email.
5. If it works, leave the scan disabled or whitelist Outlook in the antivirus settings.

Note: Windows Defender's built-in email scanning rarely causes this issue. Third-party AV tools are usually the culprit.

Fix 8: Repair the Outlook Profile

If nothing else works, the Outlook profile itself may be corrupted. Rebuilding it is a 5-minute fix.

Classic Outlook (Desktop, Windows)

1. Close Outlook.
2. Open Control Panel > Mail (Microsoft Outlook).
3. Click Show Profiles.
4. Click Add, create a new profile name (e.g., "Outlook2").
5. Set up your email account in the new profile.
6. Select Prompt for a profile to be used so you can compare before deleting the old one.
7. Open Outlook, choose the new profile, and test sending.

If the new profile works, delete the old one once you have confirmed everything is migrated.

New Outlook

New Outlook does not have classic profiles. To reset: sign out of all accounts via gear icon > Accounts > Email accounts > Manage > Remove, then sign back in.

Outlook Mobile Send Failures

On iOS and Android, the most common send-failure causes are:

1. Background app refresh disabled. Outlook may be paused. Settings > Outlook > Background App Refresh (iOS) or Battery > Background activity (Android).
2. Account needs re-auth. Outlook prompts but the prompt is sometimes missed. Open the app, tap the account icon - look for an exclamation mark or "Action needed".
3. Storage full on phone. Outlook needs free space to cache outgoing messages. Free up at least 500 MB.
4. VPN or restricted Wi-Fi. Corporate networks sometimes block SMTP. Try sending over cellular data.

Common Outlook Send Error Codes

If Outlook does throw an error, the code tells you exactly what failed:

Error code	Meaning	Fix
0x800CCC0F	Connection dropped during send	Check internet; retry
0x800CCC78	Authentication failed	Re-enter password
0x80042109	Outgoing server connection failed	Check SMTP settings
0x8004210B	Send timed out	Reduce attachment size, retry
0x80040600	Outbox file corruption	Repair the .ost or .pst file
550 5.7.1	Recipient rejected the message	Check spam policies, sender reputation

For deliverability-related rejections (550-class errors), the issue is usually email authentication. Check your SPF, DKIM, and DMARC records using a free DMARC checker or email health check.

What If a Single Recipient Is Blocking the Send?

Sometimes Outlook sends to everyone except one recipient, then the whole message bounces back. Causes:

• Bad email address. Typo in the recipient field. Check carefully.
• Recipient's mailbox is full. Their server rejects the message. They need to clear space.
• Recipient's server rejects your domain. Your sending domain has a deliverability problem - see the email authentication links above.
• Distribution list with restricted senders. If you are emailing a list like [email protected], the list admin may have restricted external senders.

Try sending the same email to a different recipient. If that works, the problem is the original recipient or their server.

For Support Teams: When Outlook Sending Issues Affect Customers

Customer support teams that send replies from Outlook are particularly exposed to sending failures - a stuck Outbox or authentication issue can delay a dozen customer replies before anyone notices. Three things help:

1. Centralize support email in a shared inbox. When ten agents are handling [email protected] from their own Outlook clients, send failures multiply. A dedicated help desk like SupportBee sends replies through one verified domain with proper authentication, reducing the surface area for these failures.
2. Monitor deliverability proactively. Use a DMARC checker and email health check tool to catch authentication issues before they cause silent send failures.
3. Standardize SMTP settings across the team. If everyone uses the same configured profile, you only have one place to fix issues. See Office 365 shared mailbox best practices for setup guidance.

For related Outlook setup, see our guides on email templates in Outlook, Outlook reminders, and Outlook auto-forwarding.

Outlook Not Sending: Quick Fix Reference

Symptom	Most Likely Cause	Fix
Email sits in Outbox in italic	Stuck send queue	Press F9 / Send/Receive
"Working Offline" in status bar	Work Offline mode on	Send/Receive tab > Work Offline (toggle off)
Password prompt keeps appearing	Lost authentication	Account Settings > Repair
Send fails on large file	Attachment over limit	Use OneDrive sharing link
New profile sends, old one does not	Profile corruption	Create new Outlook profile
550 error in reply	Recipient server reject	Check SPF/DKIM/DMARC
Mobile send delayed	Background refresh off	Enable in OS settings

Frequently Asked Questions

Why does Outlook show "sending" but never finish? The message is in the Outbox waiting to send but a connection or authentication issue is blocking it. Check Work Offline mode first, then re-authenticate your account.

How do I empty the Outlook Outbox? Open the Outbox folder, right-click each stuck message, and select Delete. If Outlook will not let you delete because it is mid-send, switch to Work Offline first.

Why is Outlook not sending emails on Mac? Same causes as Windows - offline mode, authentication, profile issues. The interface differs (System Settings instead of Control Panel), but the troubleshooting path is the same: Outlook menu > Preferences > Accounts.

Why are my Outlook emails not sending but receiving? This means your incoming (IMAP/POP) server is working but outgoing (SMTP) is not. Focus on Fix 6 (SMTP settings) - the outgoing server credentials, port, or authentication setting is likely wrong.

Will reinstalling Outlook fix send issues? Sometimes, but it is a heavy fix for what is usually a settings problem. Try creating a new profile (Fix 8) before reinstalling - it solves the same problems faster.

Can a corporate firewall block Outlook from sending? Yes. Some corporate networks block outbound port 587 or require VPN access. If sending works on personal Wi-Fi but not at the office, ask your IT admin about firewall rules.

Next Steps

Start with Fix 1 (Work Offline) - it solves the problem more often than the other seven combined. If that does not work, find your unsent message in the Outbox and use the error icon or status to identify which fix to try next. For ongoing customer support workflows, getting off individual Outlook send queues and onto a shared inbox eliminates an entire class of these failures.