SMTP Error 550 Authentication required.

When we try to send mail to a specific  external address (EARTHLINK.NET), failure delivery report returned as below:
"SMTP Protocol Returned a Permanent Error 550 Authentication required."
We are using Domino 7.02 and Outlook as Pop3 client.
Any idea on this case? Thanks.

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Hello dickchan,

1. Check if outgoing mail server authentication is required. Select that option in account settings and use same settings as incoming server

2. Your ISP may be blocking port 25. Try using alternate port 587.

If no joy, you need to use your ISP SMTP server to send out emails.

Hope this helps!

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For almost every mail server you need to authenticate to the mail server to send email externally. Otherwise the mail server(domino in this case) will see it as an attempt to relay off of it. Make sure you have the authentication settings configured correctly. You don't want to change the domino server to accept relays. Most mail servers will accept tls logins or plain text logins(base64).
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