.NET Core C#
Send HTML Email with External CSS as Related Item

Demonstrates how to compose an HTML email with an external CSS file included as a related item and referenced by CID (Content-ID).
Chilkat .NET Core C# Downloads

Download Chilkat for .NET Core C#
.NET Core C#
bool success = false;

// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// The mailman object is used for sending and receiving email.
Chilkat.MailMan mailman = new Chilkat.MailMan();

// Use your SMTP server hostname.  This example uses office365, but it could be any SMTP server..
mailman.SmtpHost = "outlook.office365.com";
mailman.SmtpPort = 587;
mailman.StartTLS = true;

// Set the SMTP login/password
mailman.SmtpUsername = "OFFICE365-SMTP-LOGIN";
mailman.SmtpPassword = "OFFICE365-SMTP-PASSWORD";

// Create a new email object
Chilkat.Email email = new Chilkat.Email();
email.Subject = "HTML Email with embedded CSS";
email.From = "Chilkat Support <my-office365-user@mydomain.com>";
email.AddTo("Chilkat Support","support@chilkatsoft.com");

Chilkat.StringBuilder sbCss = new Chilkat.StringBuilder();
bool bCrlf = true;
sbCss.AppendLine("body {",bCrlf);
sbCss.AppendLine("    background-color: powderblue;",bCrlf);
sbCss.AppendLine("}",bCrlf);
sbCss.AppendLine("h1 {",bCrlf);
sbCss.AppendLine("    color: blue;",bCrlf);
sbCss.AppendLine("}",bCrlf);
sbCss.AppendLine("p {",bCrlf);
sbCss.AppendLine("    color: red;",bCrlf);
sbCss.AppendLine("}",bCrlf);

// It's possible to add a CSS file directly by calling AddRelatedFile.
// This example will add the CSS from a string.
string filenameInHtml = "styles.css";
string contentIdCss = email.AddRelatedString(filenameInHtml,sbCss.GetAsString(),"utf-8");
if (email.LastMethodSuccess != true) {
    Debug.WriteLine(email.LastErrorText);
    return;
}

// The src attribute for the image tag is set to the contentIdCss:
Chilkat.StringBuilder sbHtml = new Chilkat.StringBuilder();
sbHtml.AppendLine("<!DOCTYPE html>",bCrlf);
sbHtml.AppendLine("<html>",bCrlf);
sbHtml.AppendLine("<head>",bCrlf);
sbHtml.AppendLine("  <link rel=\"stylesheet\" href=\"cid:CONTENT_ID_CSS\">",bCrlf);
sbHtml.AppendLine("</head>",bCrlf);
sbHtml.AppendLine("<body>",bCrlf);
sbHtml.AppendLine("",bCrlf);
sbHtml.AppendLine("<h1>This is a heading</h1>",bCrlf);
sbHtml.AppendLine("<p>This is a paragraph.</p>",bCrlf);
sbHtml.AppendLine("",bCrlf);
sbHtml.AppendLine("</body>",bCrlf);
sbHtml.AppendLine("</html>",bCrlf);

int numReplacements = sbHtml.Replace("CONTENT_ID_CSS",contentIdCss);

email.SetHtmlBody(sbHtml.GetAsString());

success = mailman.SendEmail(email);
if (success != true) {
    Debug.WriteLine(mailman.LastErrorText);
}
else {
    Debug.WriteLine("Mail Sent!");
}