Email.UseDefaultCredentials Property

Gets or sets a Boolean value that controls whether the DefaultCredentials are sent with requests. DefaultCredentials represents the system credentials for the current security context in which the application is running. Default is true.

If UserName and UserPassword is supplied, this is set to false. If UserName and UserPassword are not supplied and this is set to false then mail is sent to the server anonymously.

If you provide credentials for basic authentication, they are sent to the server in clear text. This can present a security issue because your credentials can be seen, and then used by others.