Mailbox unavailable. The server response was: Access denied - Invalid HELO name (See RFC2821 4.1.1.1)

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C#

ASP.NET

Background:
I have a website(asp.net/c#) that is hosted on a server, the website is also still on my local pc.
In my website i have a form that is used to send emails. When on my local pc and using that form to send emails it works 100% but when on the server and using that form i get the error:
Mailbox unavailable. The server response was: Access denied - Invalid HELO name (See RFC2821 4.1.1.1)

The code i use to send the email is as follows

String PasswordOfEmailAddress   = "xxx";
String Host                     = "host.com"; 


String sendFrom = "no-reply@test.co.za";
String sendTo = txtEmailAdress.Text;
String sendSubject = txtSubject.Text;
String body = txtBody.Text;

String sendMessage = body;
            
NetworkCredential emailUserPassw = new NetworkCredential(sendFrom, PasswordOfEmailAddress);
SmtpClient SendingEmail = new SmtpClient(Host);

SendingEmail.EnableSsl = true;
SendingEmail.Credentials = emailUserPassw;
SendingEmail.Send(message);            

First of all, i dont even know what the error means and second i have no idea where to start by fixing it
Someone told me to add UserDefaultCedential = false
That gave me an error of ... Server does not support sercure connections

If Anyone can help i will appreciate it