Step 1:

In this tutorial I will show how to SMTP server requires a secure connection or the client was not authenticated | Email Sending ASP.NET. Our Error is:

Step 2:

First go to your Gmail account by the right side of top menu:

Step 3:

Then in left side menu go to security:

Step 4:

Then bellow the list goes to less security app access option on:

Step 5:

You can see solve the problem.