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How to Fix Outlook Not Sending Emails

By the TechBridge team 7 min read
How to Fix Outlook Not Sending Emails

When Microsoft Outlook refuses to send emails, urgent messages stall in your Outbox and disrupt your workflow. This issue affects both the classic desktop client and the new Outlook interface on Windows, often leaving you wondering whether the problem lies with your internet connection, credentials, or email server settings.

In most cases, email delivery stops due to an oversized attachment, corrupted account credentials, conflicting third-party add-ins, or an accidental toggle of the offline mode. Here is a systematic troubleshooting guide to diagnose why your Outlook is not sending emails and resolve the bottleneck step by step.

Check Connection Status and Disable Work Offline Mode

Before modifying complex configurations, confirm that Outlook is actually connected to your mail server. Outlook features a dedicated offline mode designed to prevent sending or receiving while traveling. If this mode is accidentally toggled, messages stay in your Outbox indefinitely without displaying an immediate error prompt. Look at the bottom-right status bar in classic Outlook. If you see Disconnected, Working Offline, or Trying to connect..., your app is not communicating with the mail server. To resolve this in classic Outlook, navigate to the Send / Receive tab on the ribbon and click Work Offline to toggle it back to connected status. In the new Outlook for Windows, open Settings > General > About Outlook to confirm server connectivity, or restart your web browser if you are using Outlook on the web to rule out local Wi-Fi drops.

Clear Oversized Attachments Stuck in the Outbox

One of the most frequent reasons messages fail to leave Outlook is an attachment exceeding your email provider's file size limit. Most mail servers, including Microsoft 365, Gmail, and Yahoo, cap outgoing attachments between 20 MB and 25 MB. When you hit send on a file that exceeds the threshold, the server rejects the transmission, locking the message inside your Outbox folder and preventing subsequent emails from processing. To clear the bottleneck, switch Outlook to offline mode first by selecting Send / Receive > Work Offline. This prevents Outlook from continuously attempting to send the blocked file while you edit it. Next, open your Outbox folder in the left navigation pane. Drag the stuck email into your Drafts folder, or right-click the message and select Delete. Open the message from Drafts, remove the bulky attachment, and replace it with a cloud link via OneDrive or Google Drive. Once finished, toggle Work Offline off to resume normal transmission.

Verify Outgoing SMTP Server Settings and Authentication

If you connect an email account using IMAP or POP3 instead of an automated Microsoft Exchange configuration, incorrect Simple Mail Transfer Protocol (SMTP) settings will prevent messages from leaving your computer while still allowing incoming mail to arrive normally. This desynchronization frequently happens after a hosting provider updates security protocols or changes required ports. To inspect your outgoing configuration in classic Outlook, click File > Account Settings > Account Settings. Double-click your email account and select More Settings. On the Outgoing Server tab, ensure the checkbox labeled My outgoing server (SMTP) requires authentication is checked, and select Use same settings as my incoming mail server. Switch to the Advanced tab and verify your ports:
ProtocolPort NumberEncryption Method
SMTP (Standard Secure)587STARTTLS
SMTP (Legacy SSL)465SSL/TLS
IMAP (Incoming)993SSL/TLS
POP3 (Incoming)995SSL/TLS
Click OK and run a test account setup to confirm authentication succeeds.

Launch Outlook in Safe Mode to Isolate Faulty Add-Ins

Third-party add-ins, especially those integrated by antivirus utilities, CRM integrations, or digital signature tools, can interfere with Outlook's outgoing transport layer. Running Outlook in Safe Mode temporarily suppresses all add-ins, allowing you to isolate whether an extension is responsible for blocking message dispatches. Close Outlook entirely. Press Windows + R to open the Run dialog box, enter outlook.exe /safe, and press Enter. If prompted, select your default profile and click OK. Create a test email addressed to yourself and send it. If the message sends without hesitation, a conflicting add-in is the culprit. To identify and remove the troublesome add-in, stay in Outlook and click File > Options > Add-ins. At the bottom of the window, ensure the Manage dropdown is set to COM Add-ins and click Go. Uncheck all active add-ins, click OK, and restart Outlook normally. Re-enable your add-ins one by one while sending test emails until you find the tool causing the transmission failure.

Repair Your Outlook Data File or Rebuild the Local Cache

A damaged Outlook data file (OST or PST) can silently corrupt message queues, leaving the Outbox unable to hand over outgoing data to the spooler service. If you use an Exchange, Microsoft 365, or Outlook.com account, your local offline data is stored in an OST file. If you use a traditional POP account, it relies on a PST file. For POP accounts, you can run the built-in Microsoft Inbox Repair tool. Close Outlook and press Windows + E to open File Explorer. Navigate to C:\Program Files\Microsoft Office\root\Office16 (or your active installation path) and launch SCANPST.EXE. Browse to your PST file, click Start to scan for structural inconsistencies, and select Repair if errors appear. If you use an Exchange or IMAP account with an OST file, rebuilding the cache is safer and faster. Close Outlook, press Windows + R, type %localappdata%\Microsoft\Outlook, and press Enter. Locate the .ost file associated with your email address, rename it by adding .old to the filename, and restart Outlook. Outlook will automatically generate a fresh data file and resynchronize your mailbox directly from the server.

Update Passwords and Re-Authenticate Cloud Credentials

Modern email providers require Modern Authentication via OAuth2 rather than storing plain text passwords. When security policies expire or you update your account password on another device, Outlook may still be able to query cached inbox headers while failing to authorize new outgoing transmissions via the SMTP gateway. Open File > Office Account in classic Outlook and look under User Information. If you see a yellow warning banner prompting you to fix account credentials, click Sign In and complete your two-factor authentication prompt. If no prompt appears but sending still fails, open the Windows search bar, type Credential Manager, and open the applet. Select Windows Credentials, scroll down to Generic Credentials, and look for entries starting with MicrosoftOffice16_Data or your email provider's address. Remove these entries and relaunch Outlook, which will force a fresh, clean authentication handshake.

The bottom line

Outlook not sending emails is almost always traced to an accidental offline toggle, an oversized attachment blocking the queue, or an outdated SMTP authentication credential. By systematically working through connectivity checks, clearing your Outbox, testing in Safe Mode, and verifying server ports, you can pinpoint the roadblock and restore dependable communication without losing your drafts or message history.

Frequently asked questions

Why are my emails staying in the Outbox instead of Sent Items?

Emails stay in the Outbox when Outlook cannot establish an active connection to your outgoing mail server, if an attachment exceeds your provider's upload limit, or if an add-in blocks the sending process. Until the mail server confirms receipt, the message remains parked in the Outbox.

Can an expired password prevent sending while incoming mail still works?

Yes. Some email services use separate servers and authentication sessions for receiving (IMAP/POP) and sending (SMTP). If your outgoing authentication token expires or does not refresh properly, you may continue reading incoming messages while outgoing emails fail.

What is the maximum attachment size Outlook can send?

By default, Outlook limits attachments to 20 MB for most internet email accounts and 25 MB to 35 MB for Microsoft 365 business mailboxes. If you attempt to send files larger than this threshold, the server rejects the message. Use cloud sharing links like OneDrive for larger files.

How do I force Outlook to send queued emails immediately?

In classic Outlook, press F9 on your keyboard or open the Send / Receive tab on the ribbon and click Send/Receive All Folders. In the new Outlook, click the refresh sync icon at the top of the folder pane.

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