Content-Transfer-Encoding email header is set by Mail.dll automatically.
You can overwrite it using MimeData.ContentTransferEncoding property.
Use builder class to create s new email message:
MailBuilder builder = new MailBuilder();
builder.From.Add(new MailBox("alice@example.com", "Alice"));
builder.To.Add(new MailBox("bob@example.com", "Bob"));
builder.Subject = "Test";
builder.Text = "This is plain text message.";
MimeData attachment = builder.AddAttachment(@"c:\image.jpeg");
attachment.ContentTransferEncoding = MimeEncoding.Base64;
IMail email = builder.Create();
Finally send the message:
using (Smtp smtp = new Smtp())
{
smtp.ConnectSSL("server.example.com");
smtp.UseBestLogin("user", "password");
smtp.SendMessage(email);
smtp.Close();
}