How to Fix Outlook Not Sending Emails

When Outlook stops sending emails, it creates an immediate business problem. Emails pile up in the Outbox, the user thinks they sent them, and recipients never receive them. Contracts are delayed, client responses go missing, and the user only discovers the problem hours later when someone asks why they never replied. Outlook send failures are high-urgency tickets for good reason.

The good news is that Outlook send failures follow predictable patterns. The cause is almost always one of a handful of issues: authentication problems, attachment size limits, a stuck message in the Outbox, a corrupted Outlook profile, or a server-side block. This guide walks through each cause with step-by-step fixes.

Common Causes

• Outlook is working offline (the most overlooked cause)
• Authentication token expired or password changed
• Attachment exceeds the server's size limit
• A stuck or corrupt message blocking the Outbox queue
• SMTP server settings are incorrect or changed
• The mailbox is full and the server is rejecting outbound messages
• Antivirus or firewall blocking Outlook's connection
• Corrupted Outlook profile or data file

Step 1: Check If Outlook Is in Offline Mode

This is the most common and most embarrassing cause of Outlook send failures. Look at the bottom status bar of Outlook. If it says "Working Offline" or "Disconnected," Outlook is not communicating with the mail server at all. Emails you compose go to the Outbox but never leave the computer.

To fix this, go to the Send/Receive tab in the ribbon and click "Work Offline" to toggle it off. The status bar should change to "Connected" or show the server name. Then click "Send/Receive All Folders" to flush the Outbox. All pending messages should send within seconds.

Outlook sometimes switches to offline mode automatically when it loses network connectivity and does not always switch back when the network returns. If this happens frequently, check the network connection stability - intermittent WiFi drops or VPN disconnections are the usual trigger.

Step 2: Verify Your Credentials

If Outlook keeps prompting for a password, or if you see errors like "Authentication failed" or "Server rejected your login," the issue is credentials. This commonly happens when a password was recently changed, when multi-factor authentication (MFA) was newly required, or when an OAuth token expired.

For Microsoft 365 accounts, go to File, then Account Settings, then Account Settings again. Select your email account and click "Repair." This forces Outlook to re-authenticate with the server. You may be prompted to sign in again through the browser, which refreshes the OAuth token.

If your organization recently enabled MFA and you are using a legacy mail protocol (POP3 or IMAP with basic authentication), Outlook will stop working because basic auth does not support MFA. You need to either switch to a Microsoft 365 account type in Outlook or generate an app-specific password in your security settings.

Step 3: Check Attachment Size

Every mail server has a maximum attachment size. For Microsoft 365, the default limit is 25 MB per message (the combined size of all attachments). On-premises Exchange servers may have different limits set by the administrator. If you try to send a message with attachments exceeding the limit, it sits in the Outbox and never sends. The error message is often vague or buried in the send/receive error log.

Check the Outbox for any messages with large attachments. If you find one, open it, remove or compress the attachments, and try sending again. For files larger than the limit, use OneDrive, SharePoint, or another file-sharing service and send a link instead. Outlook for Microsoft 365 has a built-in option to automatically upload large attachments to OneDrive and share a link.

Step 4: Clear a Stuck Outbox Message

Sometimes a single corrupt message in the Outbox blocks all outbound email. Outlook processes the Outbox as a queue - if the first message fails to send, everything behind it waits. The fix is to remove the stuck message.

Switch Outlook to offline mode first (Send/Receive tab, click "Work Offline"). This prevents Outlook from trying to send while you work on the Outbox. Open the Outbox folder, find the stuck message, and either delete it or drag it back to Drafts. If you cannot open or delete it because Outlook says it is "being sent," close Outlook completely, reopen it in offline mode, and try again.

If the message still will not delete, rename the Outlook data file. Close Outlook. Navigate to the Outlook data file location (usually C:\Users\[username]\AppData\Local\Microsoft\Outlook), rename the .ost file by adding ".old" to the end, and restart Outlook. It will create a new .ost file and resync from the server. Any stuck local messages are eliminated.

Step 5: Check SMTP Server Settings

For POP3 or IMAP accounts (not Exchange or Microsoft 365), Outlook uses an SMTP server to send mail. If the SMTP settings are wrong, emails cannot be sent even though receiving works fine.

Go to File, then Account Settings, then double-click your email account. Verify the outgoing mail server (SMTP) address, port, and encryption settings. Common correct settings are: port 587 with STARTTLS, or port 465 with SSL/TLS. Port 25 is blocked by most ISPs for residential connections and many corporate firewalls.

Also check the "More Settings" button, then the "Outgoing Server" tab. Ensure "My outgoing server (SMTP) requires authentication" is checked and set to "Use same settings as my incoming mail server." Many send failures come from SMTP authentication being unchecked after a profile migration or repair.

Step 6: Check Mailbox Storage Quota

A full mailbox can block sending as well as receiving. Microsoft 365 mailboxes have a 50 GB limit by default, but administrators can set lower limits. When you hit the quota, Exchange rejects outbound messages with an error about the mailbox being full.

Check your mailbox size by going to File in Outlook and looking at the Account Information page, which shows a storage usage bar. If it is near capacity, empty the Deleted Items folder, archive old messages, and delete large attachments from sent messages. In Outlook for Microsoft 365, you can use the "Cleanup" tools under File to identify the largest items in your mailbox.

Step 7: Disable Antivirus Email Scanning

Some antivirus programs insert themselves between Outlook and the mail server to scan outbound email. When this proxy fails, breaks, or becomes incompatible with an Outlook update, email stops flowing. Norton, Avast, Kaspersky, and McAfee have all had versions that caused Outlook send/receive problems.

As a diagnostic step, temporarily disable the antivirus email scanning feature (not the entire antivirus - just the email scanning component). If Outlook immediately starts sending, you have found the problem. Either configure the antivirus to exclude Outlook, update the antivirus to the latest version, or switch to Windows Defender which does not use an email scanning proxy.

Step 8: Repair or Recreate the Outlook Profile

If none of the above steps work, the Outlook profile itself may be corrupted. Profiles store account settings, data file locations, and cached configurations. Corruption accumulates over time, especially through Outlook upgrades.

1. First try repairing: go to File, Account Settings, Account Settings, select the account, and click "Repair." Follow the wizard. This fixes many profile issues without losing settings.
2. If repair does not work, create a new profile: open Control Panel, search for "Mail," click "Show Profiles," click "Add," and set up your email account fresh. Set the new profile as default. This gives Outlook a completely clean configuration. Your email will resync from the server (for Exchange and Microsoft 365 accounts).

When to Escalate to IT Support

Escalate if:

1. Multiple users report send failures simultaneously - this indicates a server-side issue, not a client problem
2. Outlook shows errors referencing the server rejecting the connection or certificate errors - the mail server configuration has changed
3. The user's account appears to be sending spam and the server has blocked outbound mail - the account may be compromised and needs a password reset and security review
4. Send failures persist after creating a fresh Outlook profile with correct settings - the issue is server-side or network-level