a.cs

// In addition to the code below, it is necessary to either:
// 1 - Enable "Less secure app access" in Google Account -> Security
// 2 - If using 2-Step Verification, go to Google Account -> Security -> Add passwords
//   then select app = Mail and device = Windows Computer to generate a password
// An alternative to the above is to authorize access for the program as shown here:
// https://developers.google.com/gmail/api/quickstart/dotnet
using var msg         = new MailMessage();
using var attachment  = new Attachment( @"d:\icon.png" );

msg.From     = new MailAddress( "[email protected]" );
msg.Subject  = "A subject";
msg.Body     = "Hello world";

msg.To.Add( "[email protected]" );
msg.Attachments.Add( attachment );

using var client = new SmtpClient();

client.Host         = "smtp.gmail.com";
client.Port         = 587;
client.EnableSsl    = true;
client.Credentials  = new NetworkCredential( "[email protected]", "gmail-generated-password" );

try
{
  client.Send( msg );

  MessageBox.Show( "Email has been successfully sent." );
}
catch( Exception ex )
{
  MessageBox.Show( "Unable to send email. Exception: " + ex.Message );
}