In most cases, this error is reported by email clients such as Outlook. A typical message would be: 'Sending and Receiving reported error (0x800ccc78). Unable to send the message. Please verify the e-mail address in your account properties. The server responded: 5.0.0. access denied".
The first step would be to verify the credentials used to authenticate with the email (SMTP) server. Most ISPs require now all the clients to authenticate before using their SMTP server to relay email messages.
Some support forums describe this error as being recorded when there were more than one email account configured for the email client and one of them was affecting the proper authentication against the SMTP server.
M296161 describes a situation of this error being recorded when you attempt to send a message through a Mindspring Simple Mail Transfer Protocol (SMTP) server. This is caused by the fact that the Mindspring SMTP server is only accepting SMTP traffic through dialup.
For AOL customers, you may be seeing the message "Error: 0x800CCC78 The message could not be sent because the server rejected the sender's e-mail address" while using an external e-mail program because the program is using an invalid port to send e-mail.
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