In this article I will explain with an example, how to resolve the following error while sending email using MailKit in .Net.
MailKit.Net.Smtp.SmtpCommandException: Authentication failed.
Note: For more details on how to send Email using MailKit, please refer my article Send Email using MailKit in ASP.Net WebForms.
The following error occurs when you try to send email with MailKit using credentials in your .Net application.
Authentication failed
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: MailKit.Net.Smtp.SmtpCommandException: Authentication failed.
.Net MailKit Error: Authentication failed
This solution to this problem is to make sure the password entered for the Username is correct. And if the password is correct and you are using Gmail credentials then, refer this article GMAIL Error: The SMTP server requires a secure connection or the client was not authenticated.