Outlook: Error Message: An Unknown Error Has Occurred

When you attempt to send a message in Outlook Express, you may receive the following error message:

An unknown error has occurred. Account: , Server: , Protocol: SMTP, Server Response: '501 Invalid domain name', Port: 25 Secure(SSL): No, Server Error: 501, Error Number: 0x800CCC63

CAUSE
This error message can occur if any of the following conditions exists:


The Domain Name Service (DNS) host name or the DNS domain name specified in the Transmission Control Protocol/Internet Protocol (TCP/IP) properties contains invalid characters.
There is a problem with your Internet service provider's (ISP) Simple Mail Transport Protocol (SMTP) server. This is the server that is used for outgoing mail.
There are invalid characters or spaces in the Computer Name box.
The message header may be damaged.

RESOLUTION
To resolve this issue, use the appropriate method:
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:
ab12-3.com

To verify that the DNS host name and the DNS domain name contain only valid characters, follow these steps:


Click Start, point to Settings, click Control Panel, and then double- click Network.
On the Configuration tab, click TCP/IP, and then click Properties.
Click the DNS Configuration tab.
In the Host box, verify that the name contains only valid characters.
In the Domain box, verify that the name contains only valid characters.
Click OK, click OK again, and then click Yes if you are prompted to restart the computer.
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.
Change Computer Name
To change the computer name, follow these steps:


Click Start, point to Settings, and then click Control Panel.
Double-click Network, and then click the Identification tab.
Change the computer name in the Computer Name box so that it contains only any combination of letters (A-Z) and numbers (0-9). Do not use non-alphanumeric characters or blank spaces.
Click OK.
Restart Windows when you are prompted.
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