Fix Thunderbird “Sending of the Message Failed” SMTP Error

You click Send in Thunderbird, but the message stays in the Outbox and a “Sending of the message failed” alert appears. The wording can point to several different problems: Thunderbird may not reach the outgoing mail server, the server may reject your sign-in or message, or a network security tool may block the connection. The fastest way to fix it is to identify which stage failed, then change only the setting that matches the evidence.

This guide follows the current Thunderbird Account Settings workflow. Menu labels can vary slightly by release and operating system. Your email provider’s published SMTP requirements are authoritative for its server name, port, connection security, and authentication method.

What the SMTP error means

SMTP, the Simple Mail Transfer Protocol, is the method Thunderbird uses to submit outgoing mail to a provider’s server. The general error may appear with more detail, such as a timeout, authentication failure, or a server response that rejects the recipient or message. Those details lead to different fixes. A timeout usually points toward connectivity or a blocked port; an explicit sign-in failure points toward authentication; a rejection after the server receives the message can involve permissions, address policy, or size limits.

Before changing settings, check whether the email provider’s webmail can send a short message. If webmail also fails, the problem is likely on the provider account or service side. If webmail works, compare Thunderbird with the provider’s documented client settings. Mozilla’s Thunderbird sending troubleshooting guide recommends verifying the SMTP configuration and the server assigned to the account.

1. Preserve the message and read the full error

Do not delete the message from the Outbox while diagnosing; keep a copy of its text and attachments in Drafts if needed. If Thunderbird reports that the message failed before it was accepted by the server, sending it again after a fix is normally expected. If the alert is unclear or you are unsure whether the server accepted it, check Sent and webmail first so you do not send duplicate copies.

Write down the exact server name and error detail shown in the alert. Words such as timed out, authentication, relay, recipient, or message too large are useful clues. Avoid posting a screenshot publicly until you hide email addresses, usernames, server names that identify your organization, and any personal message content.

2. Confirm the account is using the right outgoing server

1. Open Thunderbird’s application menu and select Account Settings.
2. Select the affected email account in the left pane.
3. Find its Outgoing Server (SMTP) assignment. Confirm it points to the SMTP server intended for that account.
4. At the bottom of the left pane, select Outgoing Server (SMTP), choose the assigned server, and click Edit.

Accounts from different providers may each need a separate SMTP entry. A Gmail address, for example, should not usually be sent through an unrelated provider’s server. If you have several accounts, check the assignment for the account selected in the compose window; correcting the server list alone will not help if that account still points to the wrong entry.

3. Match the SMTP details to your provider’s current settings

On the Edit SMTP Server screen, compare these fields with the provider’s current setup guide:

• Server Name: the provider’s exact outgoing server hostname.
• Port: the port specified for the selected security method. Common submission ports include 587 and 465, but do not choose one by guesswork.
• Connection Security: commonly STARTTLS or SSL/TLS, as specified by the provider.
• Authentication Method: the method the provider requires, such as OAuth2 or a password-based method.
• User Name: often the full email address, though the provider defines the expected value.

Do not set Connection Security to None just to see if mail sends, and do not accept a certificate warning as a workaround. A mismatch between the port and encryption method can prevent a secure connection. Mozilla explains these controls in its Thunderbird account configuration reference.

4. Refresh authentication only after verifying the method

If the server details are correct but Thunderbird reports an authentication problem, first check the provider’s current sign-in requirements. Some services use OAuth2, which opens a provider login page and asks you to authorize Thunderbird. Others use a provider-specific password or require an administrator to enable SMTP submission. Enter credentials only in Thunderbird’s expected sign-in prompt or the provider’s trusted page.

For a work or school mailbox, ask the mail administrator whether authenticated SMTP submission is enabled for your account. On Microsoft-hosted accounts, authentication rules are changing and may differ between personal and organizational services. Mozilla’s current Microsoft OAuth guidance for Thunderbird describes these differences and notes that organization-level policy can block SMTP even when incoming mail works.

Only remove a saved SMTP password after confirming that Thunderbird is using the right server and authentication method, and only if the provider’s instructions call for a fresh sign-in. Removing the saved credential does not fix a wrong port, wrong server, or disabled SMTP permission.

5. Check the network, firewall, VPN, and TLS connection

If the error says the SMTP server timed out or cannot be reached, test a different network if practical, such as a trusted mobile hotspot. A VPN, proxy, endpoint security program, firewall, or internet service provider can block outgoing mail. Mozilla notes that some internet providers block port 25 and advises checking whether an alternate provider-supported submission port is available.

If the problem began immediately after a Thunderbird update, check the firewall or security application’s trusted-app list for the current Thunderbird version. Prefer adding Thunderbird to the allowed-app list instead of leaving protective software disabled. Also check that your computer’s date and time are correct; TLS certificate validation can fail when the system clock is substantially wrong. Never lower TLS security or add an exception to bypass a certificate error as a permanent fix.

6. Test without attachments, then check provider limits

Compose a plain-text message with a short subject and no attachments, and send it to an address you control. If that works but the original does not, try removing attachments one at a time or reducing their size. Email providers set different message-size limits, and the total encoded message can be larger than the original files. Mozilla’s large-attachment guide explains that limits vary by mail server and suggests reducing files or sharing a download link instead.

If a small test fails with a response about a recipient, relay, quota, or sending policy, the connection may already be working. Verify the recipient address and check the provider’s account status or sending restrictions. For business email, an administrator may need to review server logs or organization policies; changing Thunderbird’s port will not override a server-side restriction.

When to stop changing settings

Do not keep cycling through ports, authentication methods, or encryption settings without checking the provider’s requirements. Random combinations can weaken security, trigger account lockouts, or obscure the original cause. Likewise, do not assume that reinstalling Thunderbird will correct an SMTP server rejection; account configuration and provider policy usually remain the same after reinstalling.

If the checks above do not resolve the error, collect the Thunderbird version, operating system, provider name, exact error text, SMTP hostname, port, security mode, and authentication method. Hide the username and any tokens or credentials before sharing settings with support. Include whether webmail can send and whether the failure occurs with a small message without attachments. Mozilla recommends providing this kind of environment and server detail when asking for additional help.

Verify that sending works

After making one evidence-based change, send a short test message to an address you can check. Confirm it leaves the Outbox, appears in the expected Sent folder, and arrives in the recipient’s mailbox or webmail. Then try the original message again, checking recipient addresses and attachment size. If the test succeeds but the original is rejected, use the server’s specific rejection text to address the message or account limit instead of changing SMTP settings again.