From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

This article lists the error messages that you might see when you try to connect to an SMTP server to send mail. For each error message it identifies possible causes. For troubleshooting techniques, see: Cannot send mail

Some of these error messages are misleading. In particular, the "Please verify..." part of these messages is rarely helpful. The messages are long, making some of them look very similar. When you are troubleshooting, be careful to identify the error message exactly.

Sending mail to the SMTP server happens in stages. Different things can go wrong at each stage, so each stage has different error messages.

When a connection fails, Thunderbird provides you with up to four pieces of information about the failure. If you have to ask for support (for example, in a forum or from a helpdesk), ensure that you provide all the information:

• The status message in Thunderbird's status bar and in the Sending Messages window. This tells you the stage that Thunderbird reached when the error occurred.
• An error summary provided by Thunderbird. This is almost always: "Sending of message failed"
• The detailed error message provided by Thunderbird. This is often misleading.
• An embedded message from the server. Thunderbird includes this in its error message.

Note:  If an error message shows the server name as "localhost", it usually means that you are using a webmail extension or an anti-virus program. In this case the error might be coming from that program, not from the SMTP server.

Thunderbird errors

This section lists the stages that Thunderbird goes through when it connects to an SMTP server to send a message, and it lists the possible error messages for each stage.

Lookup

Thunderbird looks up the SMTP server's name to find its network address (IP address). The lookup service (DNS) is usually provided by your ISP as part of your Internet connection. The status message at this stage is "Looking up..."

An error occurred sending mail: Unable to connect to SMTP server server name The server may be down or may be incorrectly configured. Please verify that your Mail/News account settings are correct and try again.

• The server does not exist (but it might exist when you are connected to some other network).
• You typed the server name wrongly. (Even an extra space can make it fail.)
• Your ISP's DNS is broken. Try disconnecting and reconnecting, or contact your ISP.

This error is never caused by the server being down or incorrectly configured.

Connecting

Thunderbird connects to the SMTP server, and the SMTP server responds. The status message at this stage is "Connecting..."

The message could not be sent because connecting to SMTP server server name failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator.

or:

An error occurred sending mail: The mail server sent an incorrect greeting: message

• You specified the wrong server. The server you specified exists, but it is not an SMTP server.
• You specified the wrong port number. Ask whoever runs the SMTP server what the correct port number is.
• The server is down. This is usually temporary. If it persists, contact whoever administers the server.
• Your firewall is blocking the port.
• Your ISP is blocking the port. This often affects port 25, and you can often work around it by using port 587, but details depend on your ISP and on the SMTP server's configuration.
• See also the Security and Logging in sections, below.

An error occurred sending mail: SMTP server error. The server responded: error message Contact your mail administrator for assistance.

• In general, this indicates an error message was returned from the server, and that message is echoed as received.
• If the error message is The server responded: Syntactically invalid EHLO argument(s), you may have encountered an issue caused by a faulty hostname used as argument for the initial greeting. This problem is frequently seen with Mac OSX. Following options:
  • make sure that the hostname of your machine does not have a dot (.), underscore (_), any other special character except hyphens (-), or line breaks in it (you will have to look up the respective operating-system setting);
  • if this doesn't work, or if you can't set the hostname on your machine, see this post for instructions on how to set the greeting argument explicitly (only characters, numbers, and hyphens "-" inbetween are allowed).

Security

If your settings require it, Thunderbird creates a secure connection using TLS or SSL. The status message at this stage is "Connected..."

The message could not be sent because connecting to SMTP server server name failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator.

• You specified SSL, but the server does not support it. In this case Thunderbird makes a connection, but its attempt to use SSL times out, so the status message is "Connected...", and there is a delay (normally 60 seconds) before you see the error message.
• See also the Logging in section, below.

An error occurred sending mail: Unable to connect to SMTP server server name via STARTTLS since it doesn't support EHLO. Please verify that your Mail/News account settings are correct and try again.

or:

An error occurred sending mail: Unable to connect to SMTP server server name via STARTTLS since it doesn't offer STARTTLS In EHLO reponse. Please verify that your Mail/News account settings are correct and try again.

• You specified TLS, but the server does not support it.

Logging in

If your settings require it, Thunderbird logs in by sending your username and password. If the server does not accept the username and password combination, then Thunderbird asks you for the password again. If your password does not work, there might be various reasons:

• You typed the wrong password. Perhaps it is case sensitive.
• You specified the wrong user name. Perhaps it needs to be your entire e-mail address, or perhaps only part of your e-mail address.
• You are connecting to the wrong server, and this server does not recognize your username.
• Your user name is not yet registered on the server.
• The server is broken.

The message could not be sent because connecting to SMTP server server name failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator.

• You specified that a username and password are required, but you did not specify any username.

Identity

Thunderbird tells the server who is sending the message. This is the From address, your identity in Thunderbird.

An error occurred while sending mail. The mail server responded: error message Please verify that your email address is correct in your Mail preferences and try again.

• The server requires login with a username and password, but you did not specify this in your settings.
• The server requires security, but you did not specify this in your settings.
• The server only accepts mail from one e-mail address for your username, and you have used an identity containing a different e-mail address.

Recipients

Thunderbird tells the server who is receiving the message.

An error occurred while sending mail. The mail server responded: error message Please check the message recipients and try again.

• The server requires login with a username and password, but you did not specify this in your settings.
• The server requires security, but you did not specify this in your settings.
• There is a problem with one or more of the message recipients' e-mail addresses.

Sending

Thunderbird sends the message.

An (SMTP) error occurred while sending mail. The server responded: error message

• The server did not allow Thunderbird to start sending the message.

An error occurred while sending mail. The mail server responded: error message Please check the message and try again.

• Thunderbird sent the message, but the server failed to process it.

The size of the message you are trying to send exceeds a temporary size limit of the server. The message was not sent, try to reduce the message size and try again. The server responded: error message

or:

The size of the message you are trying to send exceeds the global size limit ( number bytes) of the server. The message was not sent, try to reduce the message size and try again.

or:

The size of the message you are trying to send exceeds the global size limit of the server. The message was not sent, try to reduce the message size and try again. The server responded: error message

• The message really is too big.
• The server is broken or badly configured.

Confirmation

The SMTP server tells Thunderbird that it received the message.

Sending of message failed. Please verify that your Mail & Newsgroups account settings are correct and try again.

• You pressed Cancel to stop the sending process. The message was probably not sent, but if you left it too late then the message might have been sent.
• The server failed to confirm that the message was sent. There is no way to tell whether the message was sent or not.

Network errors

Things can go wrong with the network at any stage in sending a message. These errors are not related to the SMTP server, but they can interrupt communication with the server, so that sending the message fails.

A communications error occurred: error message Please try again.

or:

A network error occurred while receiving data. (Network Error: error message) Try connecting again.

• There is a problem with your network connection. Try disconnecting and reconnecting. If it still does not work, contact your ISP.

Server errors

Here is a list of the standard server error codes, and brief explanations. Servers often report different messages, they often represent the code with full stops, (like 4.2.1 instead of 421), and they often use non-standard codes (for example, 571 instead of 530).

Most of these errors usually indicate faults on the server. 530 or 551 means that you are using the wrong server name, or the wrong port, or you have not specified that Thunderbird must log in with your user name and password. 554 can mean that your ISP is blocking the port.

See also

• Network tools for server connections
• Session logging for mail/news