How to set username and password for SmtpClient object in .NET?

Use NetworkCredential

Yep, just add these two lines to your code.

var credentials = new System.Net.NetworkCredential("username", "password");

client.Credentials = credentials;

The SmtpClient can be used by code:

SmtpClient mailer = new SmtpClient();
mailer.Host = "mail.youroutgoingsmtpserver.com";
mailer.Credentials = new System.Net.NetworkCredential("yourusername", "yourpassword");