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When an email gets stuck in your Outbox or returns a delivery failure, the root cause is usually an incorrect SMTP configuration, a blocked network port, an oversized attachment, or a sender authentication problem. The sections below walk you through diagnosing each cause methodically — resolve the issue at the earliest step that applies to your situation.
These steps apply to any email client (Outlook, Thunderbird, Apple Mail) unless a step specifies otherwise. If you are troubleshooting a shared mailbox or distribution list, confirm with your IT administrator that you have Send As or Send on Behalf permissions before proceeding.

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1. Check for Messages Stuck in the Outbox

Before changing any settings, confirm where in the send process the message is failing.
1

Open your Outbox folder

In Outlook, look in the folder panel on the left for the Outbox folder. A number in parentheses indicates messages waiting to be sent.
2

Open the stuck message

Double-click the message to open it. If Outlook prevents you from opening it, the message is actively attempting to send — click Send/Receive → Stop Send/Receive first.
3

Check for obvious issues

Verify that:
  • The To, CC, and BCC fields contain valid, properly formatted email addresses.
  • The message does not contain an unusually large attachment (see Step 4).
  • You are not in Work Offline mode (Send/Receive → Work Offline should not be highlighted).
4

Attempt a manual send

Press F9 or click Send/Receive → Send All to force an immediate send attempt. Note any error message that appears — this is your most useful diagnostic clue.
Error codes like 0x800CCC0F (connection dropped), 0x80042109 (cannot connect to server), or 550 5.7.x (authentication/relay rejection) each point to a specific cause. Search the error code alongside your email provider’s name for targeted guidance.

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2. Verify Your SMTP Settings

Incorrect outgoing server settings are the most common cause of send failures, especially after an account migration or password change.
1

Open Account Settings

In Outlook: File → Account Settings → Account Settings. Select your account and click Change (for IMAP/POP3) or review the Exchange settings.In Thunderbird: Account Settings → Outgoing Server (SMTP).
2

Confirm the SMTP server address

Compare your configured server against your email provider’s documented settings. Common examples:
3

Verify the port and encryption

Use the following standard port and encryption combinations:
If you are unsure which to use, try port 587 with STARTTLS first.
4

Confirm authentication settings

Ensure Require logon using Secure Password Authentication (SPA) matches your provider’s requirements. Most modern providers require standard password authentication, not SPA/NTLM.
5

Re-enter your password

Even if the password appears saved, re-enter it manually. An expired or changed password silently causes send failures without always prompting for re-entry.
Never set the SMTP port to 25 for outbound client mail submission. Most residential and corporate ISPs block port 25 outbound to prevent spam, and your messages will silently queue or fail.

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3. Test SMTP Connectivity

If your settings appear correct but sending still fails, verify that your machine can actually reach the SMTP server on the required port. A firewall, proxy, or ISP block may be silently dropping the connection.
1

Test with Test-NetConnection (Windows PowerShell)

Open PowerShell (search “PowerShell” in the Start menu) and run:
A successful result shows TcpTestSucceeded : True. If you see False, the port is blocked between your machine and the server.
2

Test with Telnet (optional, Windows)

If Telnet is enabled on your machine (Control Panel → Programs → Turn Windows features on or off → Telnet Client):
A blank screen or a 220 banner response confirms connectivity. An immediate connection error or timeout confirms a block.
3

Test from a different network

Connect your device to a mobile hotspot and attempt to send. If it succeeds, the block is at the network level (corporate firewall, ISP, or VPN) rather than on your machine.
If Test-NetConnection fails from the office network but succeeds from a hotspot, escalate to your network team to whitelist the SMTP server address and port in the firewall or proxy configuration.

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4. Check Attachment Size Limits

Emails with attachments that exceed your mail server’s size limit will permanently fail to send and may remain stuck in the Outbox without a clear error.
1

Check your provider's size limit

Common limits:
2

Check the total message size in Outlook

While composing, Outlook displays the current message size in the lower-right corner of the compose window (e.g., 18 KB). Note that file sizes increase by approximately 33% after base64 email encoding.
3

Use an alternative for large files

If your attachment exceeds the limit, upload the file to OneDrive, SharePoint, Google Drive, or another cloud storage service and share a link in the email body instead.
In Outlook with a Microsoft 365 account, clicking the attachment icon offers an option to Upload to OneDrive and share as a link automatically — this avoids size limit issues entirely.

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5. Verify Sender Authentication (SPF / DKIM)

If your messages are reaching recipients but landing in spam, or if you receive NDRs (Non-Delivery Reports) mentioning authentication failures, your domain’s SPF or DKIM records may be misconfigured. As an end user, you can verify your sending reputation without needing access to DNS settings.
1

Send a test message to mail-tester.com

Navigate to mail-tester.com in your browser. The page will display a unique test email address (e.g., [email protected]).
2

Send your test email

Compose a new email from the account you are troubleshooting and send it to the address provided by mail-tester.com. Use a realistic subject and body — not just a single word.
3

Check your score

Return to mail-tester.com and click Then check your score. The report will show:
4

Interpret and act on results

  • SPF / DKIM failures: Report to your IT administrator or email provider — these require DNS record changes.
  • Blacklist listing: Report the specific blacklist name to your IT team. Blacklist removal requests must be submitted by the domain/IP owner.
  • Content score issues: Review the email body for spam-trigger phrases or excessive links.
SPF, DKIM, and DMARC records are managed at the DNS level by your IT or hosting provider. End users cannot fix these directly. Document the mail-tester.com report and include it in your escalation ticket.

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Escalation

Escalate if any of the following apply after completing all steps above:
  • Test-NetConnection fails on both ports 587 and 465 from the corporate network, indicating a firewall rule needs to be added.
  • You receive NDRs with 550 5.7.64 or 550 5.7.1 errors, which indicate relay denial or authentication policy blocks that require server-side configuration.
  • Multiple users on the same domain are experiencing send failures simultaneously — this points to a server-side or DNS issue.
  • Your IP or domain appears on a blacklist in the mail-tester.com report.
  • SPF or DKIM checks fail — DNS record updates are required.
When logging a ticket, include:
  • The full text of any Non-Delivery Report (NDR) or bounce message.
  • The error code displayed by your email client.
  • The results of your Test-NetConnection commands.
  • Your mail-tester.com score report (screenshot or URL).