> ## Documentation Index
> Fetch the complete documentation index at: https://docs.derekdinh.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Troubleshooting Email Not Sending or Stuck in Outbox

> Fix emails stuck in Outbox by verifying SMTP settings, testing connectivity, checking size limits, and validating sender authentication.

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.

<Note>
  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.
</Note>

## 1. Check for Messages Stuck in the Outbox

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

<Steps>
  <Step title="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.
  </Step>

  <Step title="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.
  </Step>

  <Step title="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).
  </Step>

  <Step title="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.
  </Step>
</Steps>

<Tip>
  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.
</Tip>

***

## 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.

<Steps>
  <Step title="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)**.
  </Step>

  <Step title="Confirm the SMTP server address">
    Compare your configured server against your email provider's documented settings. Common examples:

    ```text theme={null}
    # Microsoft 365 / Outlook.com
    smtp.office365.com

    # Gmail
    smtp.gmail.com

    # Yahoo Mail
    smtp.mail.yahoo.com
    ```
  </Step>

  <Step title="Verify the port and encryption">
    Use the following standard port and encryption combinations:

    ```text theme={null}
    Port 587  →  STARTTLS  (recommended for most providers)
    Port 465  →  SSL/TLS   (implicit TLS, used by some providers)
    Port 25   →  Unencrypted or STARTTLS (rarely used for client submission;
                 often blocked by ISPs to prevent spam)
    ```

    If you are unsure which to use, try **port 587 with STARTTLS** first.
  </Step>

  <Step title="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.
  </Step>

  <Step title="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.
  </Step>
</Steps>

<Warning>
  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.
</Warning>

***

## 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.

<Steps>
  <Step title="Test with Test-NetConnection (Windows PowerShell)">
    Open **PowerShell** (search "PowerShell" in the Start menu) and run:

    ```powershell theme={null}
    # Test SMTP connectivity on port 587
    Test-NetConnection -ComputerName smtp.office365.com -Port 587

    # Test on port 465
    Test-NetConnection -ComputerName smtp.office365.com -Port 465
    ```

    A successful result shows `TcpTestSucceeded : True`. If you see `False`, the port is blocked between your machine and the server.
  </Step>

  <Step title="Test with Telnet (optional, Windows)">
    If Telnet is enabled on your machine (**Control Panel → Programs → Turn Windows features on or off → Telnet Client**):

    ```bash theme={null}
    telnet smtp.office365.com 587
    ```

    A blank screen or a `220` banner response confirms connectivity. An immediate connection error or timeout confirms a block.
  </Step>

  <Step title="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.
  </Step>
</Steps>

<Note>
  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.
</Note>

***

## 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.

<Steps>
  <Step title="Check your provider's size limit">
    Common limits:

    ```text theme={null}
    Microsoft 365:   25 MB per message (combined attachments)
    Gmail:           25 MB per message
    Yahoo Mail:      25 MB per message
    Exchange Server: Set by your administrator — often 10–35 MB
    ```
  </Step>

  <Step title="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.
  </Step>

  <Step title="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.
  </Step>
</Steps>

<Tip>
  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.
</Tip>

***

## 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.

<Steps>
  <Step title="Send a test message to mail-tester.com">
    Navigate to [mail-tester.com](https://www.mail-tester.com) in your browser. The page will display a unique test email address (e.g., `test-abc123@mail-tester.com`).
  </Step>

  <Step title="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.
  </Step>

  <Step title="Check your score">
    Return to mail-tester.com and click **Then check your score**. The report will show:

    ```text theme={null}
    ✔ SPF record present and valid
    ✔ DKIM signature verified
    ✔ DMARC policy found
    ✗ Your IP is listed on a blacklist
    ```
  </Step>

  <Step title="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.
  </Step>
</Steps>

<Warning>
  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.
</Warning>

***

## Escalation

<Accordion title="When to escalate to your IT Help Desk">
  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).
</Accordion>