I found now a solution!
Perhaps someone else can also use it.
I create now a eml-File and to make it "sendable" / "ready to send" you just have the add the header "X-Unsent: 1" and if you open it now you can immediate send the email :-)
here a short example:
var message = new MailMessage();
.
.
.
message.Headers.Add("X-Unsent", "1");
var client = new SmtpClient();
client.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;
client.PickupDirectoryLocation = @"C:\temp\somedirectory";
client.Send(message);
I hope this will help someone else, it was really hard to find^^