It is an error occurring due to SMTP authentication problems in C#. Additionally, you may run into this error due to a weak sender email password.
First, ensure that you've turned on SMTP authentication for your sender email. For example, Office 365 mailboxes have SMTP client authentication disabled by default. So make sure to enable SMTP authentication for your mailbox.
Then, ensure that your password cannot get guessed easily. For example, do not use "test123" as your password and use a strong password of 8 characters consisting of alphanumeric, uppercase, lowercase letters, and numeric passwords.
Finally, make sure that you are connecting using TLS, as most SMTP hosts accept this, and doing so would most likely resolve your error.
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