This is a problem that will not allow you to use the SMTP server with the script. Therefore, please turn off the use SMTP Server in Main Config. The script can still send email using the default mail() function provided in the script.
The reason for this error is that some SMTP servers require you to log in to the POP server first, before you can use the SMTP server first. (The effect is that users need to read the email before sending)
For more information, please see this external website about the POP-before-sending authentication mechanism. http://www.azc.com/htmls/faqs/pop_before_smtp.html