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However, most “Relay access denied” errors occur when a user tries to send email, but isn’t actually authenticating.
For example, if Outlook is being used to send email, and it’s using your server as a mail relay, if it doesn’t actually authenticate with your server for outgoing smtp, your server would say “relay access denied”.
Outlook, by default, doesn’t provide authentication credentials when sending Outgoing SMTP messages – that’s something that can be enabled in the Outlook settings.