Send Email in C# via SMTP – HTML Format and File Attachment

Sending emails from a C# application is a common task in business and enterprise development. Whether you're delivering a project update, sending alerts, or automating reporting workflows, reliable email functionality is essential.

In this article, we’ll walk through how to send emails in C# using Spire.Email for .NET, a powerful component that simplifies SMTP communication and email formatting. We'll explore SMTP configuration, Gmail integration, HTML and plain-text content, file attachments, and multiple recipients.

Table of Contents

• 1. Project Setup
• 2. Send a Plain-Text Email via SMTP
• 3. Send HTML Email with Attachments via Gmail SMTP
• 4. Advanced Email Features with Spire.Email
• 5. Common Errors and Troubleshooting
• 6. Use Cases for Spire.Email
• 7. Free License
• 8. FAQ

1. Project Setup

To begin, create a .NET project (Console App or ASP.NET) and add the Spire.Email for .NET package.

Install via NuGet:

Install-Package Spire.Email

Or manually download the Spire.Email package and reference the Spire.Email.dll from your local installation package.

2. Send a Basic Email via SMTP in C#

The example below shows how to use Spire.Email to send a plain-text email through an SMTP server.

using Spire.Email;
using Spire.Email.Smtp;

// Configure the SMTP client
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.example.com";
smtp.Port = 587;
smtp.Username = "your@example.com";
smtp.Password = "your_password";
smtp.ConnectionProtocols = ConnectionProtocols.StartTls;

// Create maill addresses
MailAddress sender = new MailAddress("your@example.com", "Sender Name");
MailAddress recipient = new MailAddress("recipient@example.com", "Recipient Name");
MailMessage message = new MailMessage(sender, recipient);

// Set the email content
message.Subject = "Test Email";
message.BodyText = "This is a plain-text test email.";

// Send the email
smtp.SendOne(message);

Explanation:

• Uses MailAddress and MailMessage from Spire.Email
• Establishes a secure SMTP connection using ConnectionProtocols.StartTls

The screenshot below shows the actual email received using the above SMTP code.

3. Send HTML Email with Attachments via Gmail SMTP in C#

The following C# code example demonstrates how to send an HTML-formatted email using Gmail SMTP, including multiple recipients, a file attachment, and plain-text fallback for clients that do not support HTML.

using Spire.Email;
using Spire.Email.Smtp;

// Create MailAddress objects
MailAddress from = new MailAddress("your@gmail.com", "Your Name");
MailAddress to1 = new MailAddress("user1@example.com", "User One");
MailAddress to2 = new MailAddress("user2@example.com", "User Two");

// Create a mail message
MailMessage message = new MailMessage(from, to1);
message.To.Add(to2);
message.Cc.Add(new MailAddress("cc@example.com", "CC Person"));
message.Bcc.Add(new MailAddress("bcc@example.com"));

// Set the body HTML and text for the message
message.Subject = "Monthly Report - June 2025";
message.BodyHtml = "<h2 style='color:#2E86C1;'>Report Summary</h2><p>Please see the attached PDF report.</p>";
message.BodyText = "Report Summary - please see the attached PDF.";

// Add an attachment to the email
Attachment attachment = new Attachment(@"Sample.pdf");
message.Attachments.Add(attachment);

// Configure the SMTP client
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.Username = "your@gmail.com";
smtp.Password = "your_app_password";
smtp.ConnectionProtocols = ConnectionProtocols.StartTls;

// Sen the email
smtp.SendOne(message);

Explanation:

• Uses MailAddress, MailMessage, Attachment, and SmtpClient from Spire.Email
• Combines BodyHtml and BodyText to support HTML and plain-text email clients
• Adds multiple recipients via To.Add(), Cc.Add(), and Bcc.Add()
• Sends email securely through Gmail using ConnectionProtocols.StartTls

Gmail requires App Passwords if 2FA is enabled.

The following screenshot shows the HTML-formatted email with rich text, CC and BCC recipients, and a PDF attachment, as received from Gmail SMTP.

Related article: How to Send an Email with Attachments in C#

4. Advanced Email Features with Spire.Email

This section demonstrates advanced C# email-sending capabilities using Spire.Email, including embedded images and batch dispatch.

// Prepare an email with embedded image
MailMessage message = new MailMessage(
    new MailAddress("your@domain.com", "Sender"),
    new MailAddress("client@domain.com", "Client"));

message.Subject = "Branding Update";
message.BodyHtml = "<h1>Our New Logo</h1><img src=\"cid:logo\">";
message.BodyText = "Please view this message in HTML to see the image.";

Attachment logo = new Attachment(@"C:\Images\logo.png");
logo.ContentId = "logo";
logo.DispositionType = "Inline";
message.Attachments.Add(logo);

// Prepare a second email
MailMessage message2 = new MailMessage(
    new MailAddress("your@domain.com"),
    new MailAddress("sales@domain.com"));
message2.Subject = "Weekly Summary";
message2.BodyText = "This is an automated weekly update.";

// Create SMTP client
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.domain.com";
smtp.Port = 587;
smtp.Username = "your@domain.com";
smtp.Password = "your_password";
smtp.ConnectionProtocols = ConnectionProtocols.StartTls;

// Send one email
smtp.SendOne(message);

// Send emails in batch
smtp.SendSome(new List<MailMessage> { message, message2 });

Explanation:

• Attachment.ContentId and DispositionType = "Inline" allow inline images using CID references in HTML.
• SendOne() sends a single message.
• SendSome() sends multiple messages as a batch. Ideal for scheduled reports or group notifications.

The following screenshot shows an email with an embedded company logo and a plain-text fallback, demonstrating how Spire.Email renders HTML content with inline images.

You may also like: Creating MSG Files with RTF Body and Attachments Using C#

5. Common Errors and Troubleshooting

6. Use Cases for Spire.Email

Spire.Email for .NET is suitable for:

• Daily/weekly automated reporting
• User signup and notification emails
• Order confirmations with attachments
• Form submission alerts
• Embedded-image newsletters

Its clean API removes the need to manually handle SMTP headers, MIME formatting, and encoding issues.

Conclusion

With Spire.Email for .NET, developers can quickly build robust and flexible email-sending features directly into C# applications. Whether the goal is to deliver transactional messages, generate styled reports, or dispatch scheduled newsletters, this component provides a clean, reliable API that simplifies every step of the process.

By supporting plain-text and HTML formats, attachments, Gmail SMTP, embedded images, and batch sending, Spire.Email helps streamline email integration without the overhead of external dependencies.

7. Apply for a Free Temporary License

To remove the evaluation warning in the email body or attachment footer, you can apply for a free temporary license:

Request a Free 1-Month License

Simply fill out the form and follow the instructions to apply the license in your project. This enables full feature access during evaluation.

8. FAQ

How to send HTML email in C#?

You can set the BodyHtml property of the MailMessage object to send an HTML email using Spire.Email. You can also include BodyText as a fallback for clients that don't support HTML.

How to send email to multiple recipients in C#?

Add multiple recipients using message.To.Add(), message.Cc.Add(), and message.Bcc.Add() methods. Spire.Email supports To, Cc, and Bcc fields.

How to use SMTP to send email in ASP.NET or MVC projects?

The SMTP usage in ASP.NET or MVC is the same as in Console applications. Simply configure the SmtpClient with host, port, credentials, and use the same SendOne() or SendSome() methods to dispatch your message.