How to send email appearing from alias domain? Relaying disallowed.

How to send email appearing from alias domain? Relaying disallowed.

Hi

We have a c# web form where we are trying to send an email from our alias domain - e.g. info@aliasdomain.com - however we get 

"Mailbox name not allowed. The server response was: Relaying disallowed as info@aliasdomain.com "

This info-address is a group mailbox on our Zoho account, and it is acting as an alias for our main domain, i.e. info@maindomain.com. 

I figured it is because we sign in with a main domain account like   john@maindomain.com

Our c# code is something like this:

  1.                             using (SmtpClient client = new SmtpClient())
  2.                             {
  3.                                 client.DeliveryMethod = SmtpDeliveryMethod.Network;
  4.                                 client.EnableSsl = true;
  5.                                 client.Host = "smtp.zoho.com";
  6.                                 client.Credentials = new NetworkCredential(
  7.                                     "john@maindomain.com",
  8.                                     "password123");
  9.                                 client.Port = 587;
  10.                                 client.Send(msg);
  11.                             }

Since there is no account/password details for the info-account(s) (because they are group mail accounts) how can we accomplish sending mail from webforms using them as alias?