I understand that you want to use the default browser settings, I would also like to answer this question.
Meanwhile, you can do it manually.
MailAddress from = new MailAddress("from@mailserver.com");
MailAddress to = new MailAddress("to@mailserver.com");
MailMessage mm = new MailMessage(from, to);
mm.Subject = "Subject"
mm.Body = "Body";
SmtpClient client = new SmtpClient("proxy.mailserver.com", 8080);
client.Credentials = new System.Net.NetworkCredential("from@mailserver.com", "password");
client.Send(mm);
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