The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated March 27, 2018
Posted by juanpablo1manrique in SharePoint.trackback
I have the same problem,
I resolved using
SmtpClient smtpClient = new SmtpClient(smtpServer, 25);
I discovered that the transactions try to go by the por 854,
smtpServer:854
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