The most popular protocol used for email clients for outgoing mail is the Simple Mail Transfer Protocol (SMTP). When you receive the "550" error in a mail client such as Windows Live Mail, this typically indicates that the outgoing mail server is not responding to requests. You can confirm if the server is "down" by using the "Ping" command in a Microsoft Windows Command Prompt. When the outgoing mail server is down, there is nothing you can do except to wait for the server to start responding again.
Click on the Windows "Start" button, click on "All Programs," then click on the "Windows Live Mail" shortcut icon. This will open the Windows Live Mail program.
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Right-click on your email account, on the left, then click on "Properties." This will open the account settings window in which you can ascertain the outgoing SMTP server.
Click on the "Servers" tab. Establish the SMTP server, which is located in the text field labelled "Outgoing mail (SMTP)." Make a note of the SMTP server. For explanation purposes, we shall use the example of "mail.btinternet.com."
Click on the Windows "Start" button, type "Command Prompt" in the search text field, then click on "Command Prompt," which should be the first item. This will open a Microsoft Windows Command Prompt, in which you can execute command line applications.
Type the following command without quotation marks: "ping mail.btinternet.com" and press "Enter" on your keyboard. If you see the message "Request timed out," this indicates that you cannot connect to the SMTP server, which typically indicates that the server is down. This is usually the case if the server is undergoing maintenance locally.
Wait for a few hours and try to send a message in Windows Live Mail again. Typically, mail server maintenance takes up to 24 hours to be completed.
Replace the command in Step 5 with the name of your actual mail server, for example "ping smtp.aol.com," for example.