Outlook Not Sending Emails: How to Fix It

If Outlook is refusing to send emails — or they’re sitting in your Outbox going nowhere — you’re not alone. This is one of the most common Outlook complaints, and it almost always comes down to one of a handful of causes: a server connection problem, an authentication failure, antivirus interference, or a corrupted profile. Here’s how to diagnose it and get back to a working inbox.

Step 1: Check Your Outbox First

Open your Outbox folder. If you can see emails sitting there, Outlook has tried to send them and failed. Click the Send/Receive button (or press F9) to trigger a manual send attempt and watch what error appears in the status bar or the Send/Receive Progress window.

If nothing appears in your Outbox at all, the issue may be that emails aren’t being queued — check your Drafts folder instead, as some configurations save failed sends there.

Step 2: Identify the Error Code

Outlook usually surfaces an error code when sending fails. The two most common are:

• 0x800CCC0F — Connection to server was interrupted. Usually a network timeout, firewall block, or the server dropped the connection mid-send.
• 0x80042109 — Outlook cannot connect to your outgoing (SMTP) mail server. The server settings are wrong, the port is blocked, or the server is down.

Other error messages to watch for: “Your server does not support the connection encryption type”, “Need to authenticate before sending”, and “Relaying denied”.

Step 3: Test Your Internet Connection and Server

Open a browser and visit a website. If the internet is fine, try pinging your mail server from Command Prompt:

ping smtp.office365.com

If that times out or fails, the problem is network-level — either your router, your ISP, or the mail server itself. Check Microsoft’s service status page if you’re using Microsoft 365.

Step 4: Check Your SMTP Settings

Go to File > Account Settings > Account Settings, select your email account and click Change. For Microsoft 365 and Outlook.com accounts:

• Outgoing server (SMTP): smtp.office365.com
• Port: 587 (STARTTLS) or 465 (SSL)
• Encryption: STARTTLS on port 587
• Authentication: Your full email address and password

For third-party IMAP accounts (Gmail, cPanel-hosted email, etc.), confirm your provider’s SMTP settings. Using port 25 is rarely correct for client connections — most providers block it to prevent spam relay.

Check “My Outgoing Server Requires Authentication”

In More Settings > Outgoing Server, make sure “My outgoing server (SMTP) requires authentication” is ticked and set to use the same settings as your incoming mail server. This is a common miss after account reconfiguration.

Step 5: Disable Antivirus Email Scanning Temporarily

Antivirus software — particularly older versions of Norton, AVG, and Avast — intercepts outgoing SMTP traffic and can cause Outlook to time out. Temporarily disable your antivirus email scanning (not the whole antivirus) and attempt to send a test message.

If that fixes it, look in your antivirus settings for “email protection” or “mail scanning” and either disable it permanently (Microsoft 365 handles its own email security) or whitelist Outlook as a trusted application.

Step 6: Check Windows Firewall and Port Blocking

Windows Firewall or a third-party firewall may be blocking Outlook’s outbound connections. Go to Control Panel > Windows Defender Firewall > Allow an app through Windows Defender Firewall and confirm Outlook is listed and ticked on both Private and Public networks.

To test whether port 587 is reachable, open PowerShell and run:

Test-NetConnection smtp.office365.com -Port 587

If TcpTestSucceeded returns False, the port is being blocked — either by your firewall, router, or ISP. Some ISPs block port 587; switching to port 465 with SSL often resolves this.

Step 7: Remove the Stuck Email from Outbox

Sometimes a large attachment or a corrupted message gets stuck and blocks everything behind it. To remove it:

1. Go to Send/Receive > Work Offline — this stops Outlook trying to send while you work.
2. Open the Outbox folder.
3. Right-click the stuck message and select Delete (or move it to Drafts if you want to keep it).
4. Go back to Send/Receive > Work Offline to untick it and go back online.

Putting Outlook in Work Offline mode first is critical — if you don’t, Outlook may lock the message and refuse to let you delete it.

Step 8: Fix Authentication Failures (Modern Auth)

If you’re getting “Need password” prompts that don’t go away, or authentication keeps failing even with the correct password, the issue may be Modern Authentication. Microsoft 365 now requires OAuth 2.0 (Modern Auth) rather than Basic Authentication, which was retired in 2023.

In classic Outlook (2016, 2019, 2021):

1. Go to File > Account Settings > Account Settings.
2. Select your account and click Change.
3. Click More Settings > Security.
4. Under “User Identification”, ensure OAuth Authentication is selected, not “Clear text” or “SPA”.

If you’re still stuck, remove the account from Outlook and re-add it — Outlook will negotiate Modern Auth automatically on a fresh add for Microsoft 365 accounts.

Step 9: Create a New Outlook Profile

A corrupted Outlook profile is a surprisingly common cause of persistent send failures. To create a new profile:

1. Close Outlook completely.
2. Open Control Panel > Mail (Microsoft Outlook).
3. Click Show Profiles > Add.
4. Give it a name, add your account, and set it as the default profile.
5. Open Outlook and test sending.

If sending works on the new profile, your old profile was the problem.

New Outlook (2023+) Specific Fixes

The new Outlook for Windows (the web-based version rolling out from 2023 onwards) has fewer configurable settings than classic Outlook. If you’re using new Outlook:

• Sign out and back in: Go to your account picture (top right) > Sign out, then sign back in. This refreshes your OAuth token.
• Switch back to classic Outlook: Use the toggle at the top of the new Outlook window. Classic Outlook gives you more diagnostic options.
• Check Sync settings: New Outlook syncs via Microsoft’s servers — if there’s a service issue, there’s nothing client-side you can fix. Check Microsoft’s service health dashboard.

For related issues where Outlook is connected but not updating properly, see our guide on Outlook not syncing.

Still Not Fixed?

If none of the above resolves it, run the Microsoft Support and Recovery Assistant (SaRA) — download it from Microsoft’s site, select “Outlook” and “I’m having trouble sending, receiving, or finding email”. It runs automated diagnostics and often catches things manual checks miss.

For a full index of every Outlook guide and troubleshooting fix on Serverman, see the Microsoft Outlook complete guide and troubleshooting hub.