Send Email Error: Sender domain is invalid. Could not find DNS record

Hi smile,

Sending FROM address domain is not valid.

Please recheck the address.

Or refer

https://stackoverflow.com/questions/37667252/errors-sending-emails-via-mailgun

Sending FROM address domain is not valid.

In another project, I create a frmEmail.aspx. In this page the same code is working fine. I am receiving email successfully. Code is

using (MailMessage mm = new MailMessage(txtEmail.Text.Trim(), txtTo.Text.Trim()))
{
    mm.Subject = txtSubject.Text.Trim();
    mm.Body = txtComment.Text.Trim();
    mm.IsBodyHtml = false;
    SmtpClient smtp = new SmtpClient();
    smtp.Host = "smtp.gmail.com";
    smtp.EnableSsl = true;
    NetworkCredential NetworkCred = new NetworkCredential(txtEmail.Text.Trim(), "bwlxfduacdrfcabf");//txtPassword.Text.Trim()
    smtp.UseDefaultCredentials = true;
    smtp.Credentials = NetworkCred;
    smtp.Port = 587;
    smtp.Send(mm);
    Response.Write("Email sent.");
}

but in another project it is not working and showing me error

Syntax error in parameters or arguments. The server response was: Sender domain is invalid. Could not find DNS record.

Do not use http:// or https:// prefix to your URL when configuring the Domain Name.

Example:

https://aspsnippets.com

Use aspsnippets.com

Then check.