You can always set SMTP FROM manually:
IMail email = Limilabs.Mail.Fluent.Mail.Html("Body")
.Subject("Subject")
.From("mailheaderfrom@example.net")
.To("bob@example.org")
.Create();
SmtpMail smtpMail = SmtpMail.CreateFrom(email);
smtpMail.From = "smtpfrom@example.com";
using (Smtp smtp = new Smtp())
{
smtp.ConnectSSL("smtp.example.net");
smtp.UseBestLogin("smtpuser", "password");
smtp.SendMessage(smtpMail);
smtp.Close();
}
Please note that your SMTP server might not like that MAIL FROM, From header and SMTP user are different.
As a side note, do you know VERP:
https://www.limilabs.com/blog/verp-variable-envelope-return-path-net