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Outlook Not Sending Emails: 8 Fixes for Stuck Messages in Outbox

Last updated: March 28, 2026

By Sufyan Khan — Founder, FixThatApp  |  Editorial standards

You hit Send, watch the email disappear from Drafts — and then nothing. It is sitting in the Outbox, the progress bar spins but never completes, or you eventually get a delivery failure notice in your inbox. Meanwhile you are wondering whether the recipient got your message, whether you should resend it and risk duplicates, and what is actually wrong with Outlook.

Outlook email sending failures have a handful of well-defined causes. The most common by a wide margin is Outlook silently switching to offline mode. After that, oversized attachments, SMTP configuration issues, and add-in conflicts account for nearly all remaining cases. This guide covers every scenario with specific step-by-step fixes for Outlook desktop, Outlook on the web, and the mobile app.

Quick Diagnosis: Identify Your Specific Symptom

Email sits in Outbox and never sends → Almost certainly Offline Mode. Check the status bar at the bottom of Outlook for "Working Offline" text. Fix 1 covers this.

You get a bounce-back / NDR email → The email left your Outbox but was rejected by a server. Check the error code in the NDR. Fix 5 and Fix 8 apply.

Outlook says "Not Responding" when sending → An add-in is likely hanging the send process. Fix 6 (disable add-ins) is your first step.

Email disappears but recipient never got it → Check Sent Items. If it is there, the issue is with the recipient's mail server or spam filtering. If it is not in Sent Items, check the Outbox.

Large attachment causes the send to fail → Attachment over the server size limit. See Fix 4.

Sends fine on webmail but not desktop Outlook → Local installation issue. See Fix 6 and Fix 7.

Fix 1: Check if Outlook is in Offline Mode

Why this works: Outlook has a "Work Offline" toggle that disconnects it from the mail server while keeping the application open. When this is active, emails queue in the Outbox indefinitely. The status bar at the bottom of the Outlook window shows "Working Offline" in plain text, but many people overlook it entirely.

  1. Look at the bottom status bar of the Outlook window. If you see "Working Offline" displayed, offline mode is the cause.
  2. Click the Send / Receive tab in the ribbon at the top.
  3. Click Work Offline — if it appears highlighted or pressed, clicking it will toggle online mode back on.
  4. The status bar should change to "Connected" or disappear entirely.
  5. Outlook will automatically attempt to send any queued Outbox messages within a few seconds.

If Outlook keeps switching back to offline mode on its own, check your internet connection and verify that your Exchange or Microsoft 365 server is reachable. A dropped VPN connection or network change can trigger automatic offline switching.

Fix 2: Check and Clear the Outbox Manually

Why this works: A single stuck email can block the entire Outbox queue. Outlook processes the Outbox sequentially, so if one message is locked, nothing behind it will send. You need to address the stuck message first.

  1. Click Send / Receive tab → click Work Offline to go offline (this prevents Outlook from locking the email while you try to delete it).
  2. Navigate to the Outbox folder in your folder list.
  3. Right-click the stuck message and choose Delete to remove it, or double-click it to open and edit it.
  4. Once dealt with, click Work Offline again to toggle back to online mode.
  5. Click Send All in the Send / Receive tab to flush any remaining queued messages.

Note: You must go offline before trying to delete a stuck email, otherwise Outlook will prevent deletion because it is "in use" during the send attempt.

Fix 3: Verify Your SMTP Server Settings

Why this works: If your email provider changed their SMTP server address, port, or authentication requirements, your existing Outlook settings become wrong and no emails will send. This often happens silently — everything looks normal in Outlook but sending fails at the server level.

  1. Go to File → Account Settings → Account Settings.
  2. Select your email account and click Change.
  3. Click More Settings → Outgoing Server tab. Confirm "My outgoing server (SMTP) requires authentication" is checked.
  4. Go to the Advanced tab. Verify the SMTP port: port 587 with STARTTLS is standard for most providers. Port 465 with SSL is also common. Port 25 is typically blocked by ISPs.
  5. Compare your settings against your email provider's official documentation (look for their SMTP configuration page).
  6. Click Test Account Settings to verify the connection works before closing.

Fix 4: Check Email and Attachment Size Limits

Why this works: Every email server enforces a maximum message size. Microsoft 365 defaults to 25 MB for outbound messages. Exchange Server limits vary by organization. If your email with attachments exceeds the limit, it will sit in the Outbox permanently with no obvious error message displayed to the user.

  1. Open the stuck message in the Outbox (go offline first as described in Fix 2).
  2. Check the total size of all attachments. If they total more than 20 MB, the message may be hitting the limit.
  3. Remove large attachments from the email.
  4. Upload files to OneDrive, Google Drive, or Dropbox and share a link in the email body instead. This is best practice regardless of size limits.
  5. If you need to confirm your organization's specific limit: check with your IT administrator or look at the Microsoft 365 admin portal under Exchange settings.

Fix 5: Check Microsoft 365 Service Health

Why this works: Microsoft 365 Exchange Online has periodic outages that prevent emails from being sent or received. If the problem started suddenly and affects multiple users in your organization, the cause is likely a service incident rather than a local configuration issue.

  1. If you are an IT administrator, go to admin.microsoft.com → Health → Service health. Active incidents are listed here.
  2. If you are not an admin, visit downdetector.com/status/office-365 or check the Microsoft 365 Status Twitter/X account (@MSFT365Status) for real-time incident reports.
  3. You can also try sending from outlook.office.com (web version) — if webmail also fails to send, the issue is with Microsoft's servers. If webmail sends fine, the issue is your desktop Outlook installation.

Fix 6: Disable Outlook Add-ins Temporarily

Why this works: Third-party add-ins (Grammarly, Zoom, Adobe, antivirus email scanners, CRM plugins) plug into Outlook's send process. A buggy or outdated add-in can intercept and halt the send operation entirely, causing Outlook to freeze or become "not responding" when you try to send.

  1. Go to File → Options → Add-ins.
  2. At the bottom, next to "Manage:", ensure COM Add-ins is selected and click Go.
  3. Uncheck all add-ins to disable them all at once.
  4. Close and reopen Outlook, then try sending a test email.
  5. If sending works with add-ins disabled, re-enable them one at a time to identify which one is the culprit. Update or remove the problematic add-in.

Fix 7: Repair Your Office Installation

Why this works: Office application files can become corrupted, particularly after Windows updates, failed Office updates, or disk errors. Microsoft provides a built-in repair tool that fixes corrupted installation files without uninstalling Office or affecting your documents.

  1. Close Outlook completely.
  2. Open Settings → Apps → Installed apps (Windows 11) or Control Panel → Programs → Programs and Features (Windows 10).
  3. Find Microsoft Office or Microsoft 365 in the list.
  4. Click it and select Modify.
  5. Choose Quick Repair first — this runs entirely offline and takes a few minutes. If the problem persists, run Online Repair, which downloads fresh files and takes longer but is more thorough.
  6. Reopen Outlook and test sending after the repair completes.

Fix 8: Review Email Block Lists and Sending Rules

Why this works: If you are getting Non-Delivery Reports (NDRs) for specific recipients, the issue may not be with Outlook itself but with rules or blocks affecting specific email addresses or domains. This is particularly common in corporate Exchange environments.

What NOT to Do

Common mistakes that make this worse
  • Don't repeatedly click Send when an email is stuck in Outbox. Each click queues another attempt and can create duplicate drafts. Open the Outbox, delete the stuck message, fix the underlying issue, then resend.
  • Don't ignore the difference between SMTP and IMAP errors. 'Cannot connect to server' means IMAP (incoming) is down. 'Message could not be sent' means SMTP (outgoing) has a problem. These require different fixes.
  • Don't remove and re-add your account as a first step. Removing an account deletes all locally cached emails and forces a full re-sync. For most sending failures there is a targeted fix in Account Settings that doesn't wipe local data.
  • Don't test by sending email to yourself. Outlook often delivers internally even when external SMTP is broken. Send to a different domain (Gmail, etc.) to get an accurate test.

Frequently Asked Questions

Q: Why do Outlook emails get stuck in the Outbox?

A: Emails get stuck in the Outbox for several reasons: Outlook is in offline mode (most common), an attachment exceeds the server size limit, SMTP server credentials have changed, or an add-in is interfering with the send process. Check the status bar at the bottom of Outlook for "Working Offline" — if you see it, that is almost certainly the cause.

Q: How do I delete an email that is stuck in the Outbox?

A: Outlook locks stuck emails while attempting to send them. To delete a stuck Outbox email: first go offline via Send/Receive tab → Work Offline, then open the Outbox folder, right-click the stuck message and choose Delete. Then toggle Work Offline off again to reconnect.

Q: What does "Your message did not reach some or all of the intended recipients" mean?

A: This Non-Delivery Report means your email was rejected after leaving Outlook but before being delivered. Common causes: the recipient's address does not exist, their mailbox is full, your IP is on a block list, or the recipient's server rejected your message due to content filtering. The NDR usually includes a specific error code that reveals the exact cause.

Q: Outlook sends emails fine from webmail but not the desktop app — why?

A: If webmail sends fine but the desktop Outlook cannot, the issue is with your local Outlook installation, not your email account or server. Focus on Fix 6 (disable add-ins) and Fix 7 (repair Office). A corrupted Outlook profile is another possibility — create a new profile via Control Panel → Mail → Show Profiles → Add.

Still Stuck?

If none of the fixes above resolve the issue, create a new Outlook profile as a test: go to Control Panel → Mail → Show Profiles → Add and configure your account fresh. If the new profile sends fine, your original profile is corrupted and should be rebuilt. For Microsoft 365 business accounts, contact your IT administrator or open a support ticket at admin.microsoft.com. For personal accounts, visit support.microsoft.com.

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