OL2000: Error Message "An unknown error was returned from the SMTP server."

This article was previously published under Q223477
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When you attempt to send e-mail using your Internet Service Provider (ISP), you receive an error similar to the following:
An unknown error was returned from the SMTP server.
The error message you receive may be more complex than this. See the "Examples of Complete Errors Messages" section.
NOTE: Microsoft Office 2000 has built-in functionality that allows you to get more information about difficult-to-troubleshoot alerts or error messages. If you want to enable this functionality for this and other error messages in Microsoft Office 2000, please download the Microsoft Office 2000 Customizable Alerts file from the Microsoft Office Update Web site at the following address:

NOTE: If you reached this article by clicking the Web Info button in an error message, you already have Customizable Alerts enabled.



Sending Mail Through Another Provider's SMTP Gateway.

This problem affects all messaging clients, regardless of manufacturer.

Customers should contact their ISP for assistance since this problem is not caused by the e-mail client. From a messaging client standpoint, there are no solutions for customers whose ISP's block all SMTP Relay traffic.

Most of the new SMTP e-mail gateway software can allow relaying for specific IP addresses. This is how cable modem and ADSL users will be able to continue to use their same ISP. The cable modem or ADSL provider must provide the customer a static IP address, in turn, the customer's e-mail ISP will allow relaying for that specific IP address. Banks of IP addresses can also be allowed, which may provide a workaround for some corporate customers who access ISP's through their corporate LAN.

More Information on Relay Blocking

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DNS Host or Domain Name Contains Invalid Characters

Verify that the DNS host name and the DNS domain name contain only valid characters. Valid characters for the DNS host name and the DNS domain name are letters (a-z), numerals (0-9), and the hyphen (-). The period (.) is also a valid character when it is used as a separator. For example:
To verify that the DNS host name and the DNS domain name contain only valid characters, follow these steps:
  1. Click Start, point to Settings, click Control Panel, and then double-click Network.
  2. On the Configuration tab, click TCP/IP and then click Properties.
  3. Click the DNS Configuration tab.
  4. In the Host box, verify that the name contains only valid characters.
  5. In the Domain box, verify that the name contains only valid characters.
  6. Click OK, click OK again, and then click Yes if you are prompted to restart the computer.
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Problem with SMTP Server

To determine if there is a problem with your ISP's SMTP server, wait for a period of time and then try to send the message again. If you still receive the error message, contact your ISP.

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Change the Computer Name

To change the computer name, follow these steps:
  1. Click Start, point to Settings, and then click Control Panel.
  2. Double-click Network, and then click the Identification tab.
  3. Change the computer name in the Computer Name box so that it contains only a combination of letters (a-z) and numbers (0-9). Do not use non-alphanumeric characters or blank spaces.

    NOTE: This error has been known to be generated by having an apostrophe (') in the computer name.
  4. Click OK.
  5. Restart Windows when you are prompted.
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Message or Attachment Exceeds the Allowable Size

Contact your ISP to see if another server is available which will handle your message and attachment.

Some ISP companies provide two Simple Mail Transfer Protocol (SMTP) addresses, one for most messages and a secondary one for handling larger attachments. Many ISPs will automatically switch between these servers based on need. If your ISP does not switch automatically or will not allow you to connect to the secondary server on a default basis, you may need to set up a second profile or account to address the secondary server for sending larger messages.

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Miscellaneous Conditions or Configuration Issues

For additional information about troubleshooting e-mail sending issues, please see the following article in the Microsoft Knowledge Base:
195922 OL2000: How to Troubleshoot Mail Stuck in the Outbox

Examples of Complete Error Messages

The errors received may be more complex than the generic message listed in the Error Message section.

An example of an error you may receive if you are relaying messages through an alternate SMTP server is:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was '<someone@microsoft.com>'.Subject: '<Test>', Account: '<Test>', Server:'<smtp.microsoft.com>', Protocol: SMTP, Server Response: '550 <someone@microsoft.com>... Relaying Denied', Port: 25, Secure (SSL): No, Server Error: 550, Error Number: 0x800CCC79
An example of an error message you may receive when a DNS or TCP/IP configuration issue is the cause:
An unknown error has occurred. Account: <acct>, Server: <srv>, Protocol: SMTP, Server Response: '501 Invalid domain name', Port: 25 Secure(SSL): No, Server Error: 501, Error Number: 0x800CCC63
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Background on Relay Blocking

This problem may become more common as ISP's begin to take action against junk e-mail or spam. Junk e-mail is flooding the Internet with many copies of the same message in an attempt to send the message to people who would not otherwise choose to receive it. Most spam uses an automated system to send commercial advertising, or mass marketing e-mail messages. Spam costs the sender very little to send; most of the costs are paid by the recipient or the e-mail providers.

Some ISP's are fighting spam by requiring each user to be dialed directly into their system in order to send e-mail through their SMTP gateway. This is an easy way to control spam but may affect some users who tunnel into their e-mail account from another ISP.

MSN, the Microsoft Network is one example of an ISP that has imposed spam blocking. Additionally, MSN has imposed restrictions whereby if the e-mail address in your Internet Account properties does not match that of your MSN account, you may also receive SMTP blocking errors. This affects users who receive mail with two different Internet addresses but want the same reply address on both e-mail accounts. Other ISP's implement similar strategies.


  • Employees have personal ISP accounts at isp.com where isp.com is your Service Provider's domain name. They use the company's corporate LAN to access their isp.com e-mail. They have no problems sending or receiving. One day they start receiving SMTP blocking errors when they try to reply to e-mail that they have received from isp.com. They contact isp.com technical support and are told that isp.com just installed new SMTP gateway software which prevents SMTP relaying. Now the employees can receive e-mail through the LAN, but cannot send e-mail because they're not connected directly to isp.com.
  • A user has two ISP accounts, one with isp.com and one with myisp.net where isp.com and myisp.net are the service providers' domain names. The user dials into myisp.net and is able to both send and receive mail from both accounts without problems. One day the user connects to myisp.net, tries to send mail through isp.com and receives blocking errors. The user connects directly toisp.com and can still send and receive mail with both accounts. In this example, isp.com implemented SMTP relay blocking and myisp.net did not.
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More information
For additional information about troubleshooting message sending errors in Outlook, please see the following article(s) in the Microsoft Knowledge Base:
223462 OL2000: Troubleshooting the Error 'Could not send the message.'
223465 Troubleshooting the Error "Unable to send this item."

Article ID: 223477 - Last Review: 12/05/2015 13:36:53 - Revision: 4.0

Microsoft Outlook 2000 Standard Edition

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