In this article I will explain with an example, how to send email with multiple attachments using MailKit library in ASP.Net Core (.Net Core) MVC.
Note: For beginners in ASP.Net Core (.Net Core 7), please refer my article ASP.Net Core 7: Hello World Tutorial with Sample Program example.
 
 

Installing MailKit package

You will need to install the MailKit package, for details on installation please refer Install MailKit from Nuget in Visual Studio.
 
 

Mail Server Settings in AppSettings.json file

The mail server settings are saved in the SMTP section as shown below.
{
 "Smtp": {
    "Server": "smtp.gmail.com",
    "Port": 587
 }
}
 
 

MimeKit MimeMessage and MailKit SmtpClient class

MimeMessage class properties

Following are the required properties of the MimeMessage class.
From – Sender’s email address.
To – Recipient(s) Email Address.
CC – Carbon Copies. (If any)
BCC – Blind Carbon Copies. (If any)
Subject – Subject of the Email.
Body – Body of the Email.
Attachments – Attachments. (If any)
ReplyTo – ReplyTo Email address.
 

SmtpClient class methods

Following are the methods of the SmtpClient class.
Connect – The connection to the SMTP Server is established using the domain and the port number.
Authenticate – The username and password of the SMTP Server is authenticated.
Send – The MimeMessage object is passed to it and the email is sent.
Disconnect – Disconnects the connection with SMTP Server.
 
 

Model

The Model class consists of the following properties.
Note: IFormFile is the new Class for Files in .Net Core. It is a replacement of HttpPostedFileBase class.
 
public class EmailModel
{
    public string To { get; set; }
    public string Subject { get; set; }
    public string Body { get; set; }
    public List<IFormFile> Attachments { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
}
 
 

Namespaces

You will need to import the following namespaces.
using MimeKit;
using MailKit.Net.Smtp;
using MailKit.Security;
 
 

Controller

The Controller consists of following Action methods.

Action method for handling GET operation

Inside this Action method, simply the View is returned.
 

Action method for handling POST operation

This Action method handles the call made from the POST function from the View.
Note: For more details on how to use Model class object for capturing Form field values, please refer my article ASP.Net Core: Form Submit (Post) Example.
 
Inside this Action method, the posted values are captured through the EmailModel class object. All the fetched values are set into an object of the MailMessage class except the Body and Attachments.

Setting Body of Email

For Body and Attachments, an object of Builder class is created. The Body of the email is Text (Non HTML) hence it is set into the TextBody property of the Builder class object.

Attaching multiple Files

The IFormFile is checked for Attachments and if it has Files then a FOR EACH loop is executed over the posted Files and inside the loop each File is added as Attachment to the Builder object.

Sending Email

Then, an object of the SmtpClient class is created and the values of Host and Port are fetched from the Smtp section of the AppSettings.json file and are passed as parameter to the Connect method along with the SecureSocketOptions set to Auto.
Note: Setting Auto allows MailKit service to decide which SSL or TLS option to use. Hence, enabling SSL is not required.
 
Also, the CheckCertificateRevocation is set to FALSE.
Note: This is optional and must be used only when Certificate errors are occurring.
 
Finally, the methods of the Mail Server such as Connect, Authenticate, Send and Disconnect are executed and a success message is set into the ViewBag object.
public class HomeController : Controller
{
    public IConfiguration Configuration { get; set; }
 
    public HomeController(IConfiguration _configuration)
    {
        this.Configuration = _configuration;
    }
 
    public IActionResult Index()
    {
        return View();
    }
 
    [HttpPost]
    public IActionResult Index(EmailModel model)
    {
        //Read SMTP section from AppSettings.json.
        string host = this.Configuration.GetValue<string>("Smtp:Server");
        int port = this.Configuration.GetValue<int>("Smtp:Port");
 
        using (MimeMessage mm = new MimeMessage())
        {
            mm.From.Add(new MailboxAddress("Sender", model.Email));
            mm.To.Add(new MailboxAddress("Recipient", model.To));
            mm.Subject = model.Subject;
            BodyBuilder builder = new BodyBuilder()
            {
                TextBody = model.Body
            };
            if (model.Attachments != null)
            {
                foreach (IFormFile attachment in model.Attachments)
                {
                    string fileName = Path.GetFileName(attachment.FileName);
                    builder.Attachments.Add(fileName, attachment.OpenReadStream());
                }
            }
            mm.Body = builder.ToMessageBody();
            using (SmtpClient smtp = new SmtpClient())
            {
                //Set to False to avoid Certificate verification.
                smtp.CheckCertificateRevocation = false;
                smtp.Connect(host, port, SecureSocketOptions.Auto);
                smtp.Authenticate(model.Email, model.Password);
                smtp.Send(mm);
                smtp.Disconnect(true);
            }
        }
        ViewBag.Message = "Email Sent.";
        return View();
    }
}
 
 

View

HTML Markup

Inside the View, the EmailModel class is declared as Model for the View and ASP.Net TagHelpers is inherited.
The View consists of an HTML Form which has been created using the following TagHelpers attributes.
asp-action – Name of the Action. In this case the name is Index.
asp-controller – Name of the Controller. In this case the name is Home.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
The HTML Form has been specified with enctype=“multipart/form-data” attribute as it is necessary for File Upload operation
The Form consists of HTML TextBox, TextArea, FileUpload element and a Submit Button.
The FileUpload element has been set with the multiple attribute for selecting multiple Files at a time.
 

Submitting the Form

When the Submit Button is clicked, the Form gets submitted and the Model object is sent to the Controller.
Finally, the ViewBag object is checked for NULL and if it is not NULL then the value of the object is displayed using JavaScript Alert MessageBox.
@model Multiple_Attachments_MailKit_Core_MVC.Models.EmailModel
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@{
    Layout = null;
}
 
<!DOCTYPE html>
 
<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
</head>
<body>
    <form method="post" asp-controller="Home" asp-action="Index" enctype="multipart/form-data">
        <table border="0" cellpadding="0" cellspacing="0">
            <tr>
                <td style="width: 80px">To:</td>
                <td><input type="text" asp-for="To" /></td>
            </tr>
            <tr>
                <td>Subject:</td>
                <td><input type="text" asp-for="Subject" /></td>
            </tr>
            <tr>
                <td valign="top">Body:</td>
                <td><textarea cols="20" rows="3" asp-for="Body"></textarea></td>
            </tr>
            <tr>
                <td>File Attachment:</td>
                <td><input type="file" asp-for="Attachments" multiple="multiple" /></td>
            </tr>
            <tr>
                <td>Gmail Email:</td>
                <td><input type="text" asp-for="Email" /></td>
            </tr>
            <tr>
                <td>Gmail Password:</td>
                <td><input type="password" asp-for="Password" /></td>
            </tr>
            <tr>
                <td></td>
                <td><input type="submit" value="Send" /></td>
            </tr>
        </table>
    </form>
    @if (ViewBag.Message != null)
    {
        <script type="text/javascript">
            window.onload = function () {
                alert("@ViewBag.Message");
            };
        </script>
    }
</body>
</html>
 
 

Possible Errors

The possible errors (exceptions) occurring while sending email with MailKit in .Net are covered in the following article.
Errors (Exceptions) while sending email with MailKit in .Net.
 
 

Screenshots

Email Form

ASP.Net Core: Send Email with multiple attachments using MailKit
 

Received Email

ASP.Net Core: Send Email with multiple attachments using MailKit
 
 

Downloads

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