An existing connection was forcibly closed by the remote host

Thank you for your patience. Regarding the issue with SendGrid in your C# ASP.NET WebForm project, the error "An existing connection was forcibly closed by the remote host" typically indicates a problem with SSL/TLS settings or network connectivity.

Here’s a revised approach to address this issue:

1. Update SendGrid Package: Ensure that you are using the latest version of the SendGrid NuGet package. Sometimes, updating to the latest version can resolve SSL/TLS issues.

2. Check SSL/TLS Settings: Verify that your application is configured to use the appropriate SSL/TLS version. You can enforce TLS 1.2 by adding the following code to your application startup

   ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
   

    

3. Verify API Key: Double-check your SendGrid API key to ensure it's correct and has not expired or been revoked.

4. Inspect Firewall and Proxy Settings: Ensure that your network firewall or proxy settings are not blocking the connection to SendGrid's servers.

5. Here’s a sample code snippet incorporating these suggestions:

   protected async void btnSend_Click(object sender, EventArgs e)
   {
       emailfrom = txtEmail.Text;
       sub = txtSubject.Text;
       body = txtBody.Text;
       ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; // Ensure TLS 1.2
       await Execute().ConfigureAwait(false);
   }
   
   static async Task Execute()
   {
       var apiKey = "my key"; 
       var client = new SendGridClient(apiKey);
       var from = new EmailAddress(emailfrom, "someone");
       var subject = sub;
       var to = new EmailAddress("[email protected]", "venus");
       var plainTextContent = body;
       var htmlContent = "" + body + "";
       var msg = MailHelper.CreateSingleEmail(from, to, subject, plainTextContent, htmlContent);
       var response = await client.SendEmailAsync(msg).ConfigureAwait(false);
       System.Diagnostics.Debug.WriteLine(response.StatusCode);
       System.Diagnostics.Debug.WriteLine(await response.Body.ReadAsStringAsync());
   }